mirror of https://github.com/k3s-io/k3s
275 lines
13 KiB
Markdown
275 lines
13 KiB
Markdown
# OCI Image Configuration
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An OCI *Image* is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime.
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This specification outlines the JSON format describing images for use with a container runtime and execution tool and its relationship to filesystem changesets, described in [Layers](layer.md).
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This section defines the `application/vnd.oci.image.config.v1+json` [media type](media-types.md).
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## Terminology
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This specification uses the following terms:
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### [Layer](layer.md)
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* Image filesystems are composed of *layers*.
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* Each layer represents a set of filesystem changes in a tar-based [layer format](layer.md), recording files to be added, changed, or deleted relative to its parent layer.
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* Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer.
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* Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if they were one cohesive filesystem.
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### Image JSON
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* Each image has an associated JSON structure which describes some basic information about the image such as date created, author, as well as execution/runtime configuration like its entrypoint, default arguments, networking, and volumes.
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* The JSON structure also references a cryptographic hash of each layer used by the image, and provides history information for those layers.
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* This JSON is considered to be immutable, because changing it would change the computed [ImageID](#imageid).
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* Changing it means creating a new derived image, instead of changing the existing image.
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### Layer DiffID
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A layer DiffID is the digest over the layer's uncompressed tar archive and serialized in the descriptor digest format, e.g., `sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9`.
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Layers SHOULD be packed and unpacked reproducibly to avoid changing the layer DiffID, for example by using [tar-split][] to save the tar headers.
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NOTE: Do not confuse DiffIDs with [layer digests](manifest.md#image-manifest-property-descriptions), often referenced in the manifest, which are digests over compressed or uncompressed content.
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### Layer ChainID
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For convenience, it is sometimes useful to refer to a stack of layers with a single identifier.
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While a layer's `DiffID` identifies a single changeset, the `ChainID` identifies the subsequent application of those changesets.
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This ensures that we have handles referring to both the layer itself, as well as the result of the application of a series of changesets.
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Use in combination with `rootfs.diff_ids` while applying layers to a root filesystem to uniquely and safely identify the result.
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#### Definition
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The `ChainID` of an applied set of layers is defined with the following recursion:
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```
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ChainID(L₀) = DiffID(L₀)
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ChainID(L₀|...|Lₙ₋₁|Lₙ) = Digest(ChainID(L₀|...|Lₙ₋₁) + " " + DiffID(Lₙ))
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```
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For this, we define the binary `|` operation to be the result of applying the right operand to the left operand.
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For example, given base layer `A` and a changeset `B`, we refer to the result of applying `B` to `A` as `A|B`.
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Above, we define the `ChainID` for a single layer (`L₀`) as equivalent to the `DiffID` for that layer.
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Otherwise, the `ChainID` for a set of applied layers (`L₀|...|Lₙ₋₁|Lₙ`) is defined as the recursion `Digest(ChainID(L₀|...|Lₙ₋₁) + " " + DiffID(Lₙ))`.
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#### Explanation
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Let's say we have layers A, B, C, ordered from bottom to top, where A is the base and C is the top.
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Defining `|` as a binary application operator, the root filesystem may be `A|B|C`.
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While it is implied that `C` is only useful when applied to `A|B`, the identifier `C` is insufficient to identify this result, as we'd have the equality `C = A|B|C`, which isn't true.
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The main issue is when we have two definitions of `C`, `C = C` and `C = A|B|C`.
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If this is true (with some handwaving), `C = x|C` where `x = any application`.
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This means that if an attacker can define `x`, relying on `C` provides no guarantee that the layers were applied in any order.
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The `ChainID` addresses this problem by being defined as a compound hash.
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__We differentiate the changeset `C`, from the order-dependent application `A|B|C` by saying that the resulting rootfs is identified by ChainID(A|B|C), which can be calculated by `ImageConfig.rootfs`.__
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Let's expand the definition of `ChainID(A|B|C)` to explore its internal structure:
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```
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ChainID(A) = DiffID(A)
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ChainID(A|B) = Digest(ChainID(A) + " " + DiffID(B))
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ChainID(A|B|C) = Digest(ChainID(A|B) + " " + DiffID(C))
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```
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We can replace each definition and reduce to a single equality:
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```
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ChainID(A|B|C) = Digest(Digest(DiffID(A) + " " + DiffID(B)) + " " + DiffID(C))
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```
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Hopefully, the above is illustrative of the _actual_ contents of the `ChainID`.
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Most importantly, we can easily see that `ChainID(C) != ChainID(A|B|C)`, otherwise, `ChainID(C) = DiffID(C)`, which is the base case, could not be true.
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### ImageID
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Each image's ID is given by the SHA256 hash of its [configuration JSON](#image-json).
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It is represented as a hexadecimal encoding of 256 bits, e.g., `sha256:a9561eb1b190625c9adb5a9513e72c4dedafc1cb2d4c5236c9a6957ec7dfd5a9`.
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Since the [configuration JSON](#image-json) that gets hashed references hashes of each layer in the image, this formulation of the ImageID makes images content-addressable.
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## Properties
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Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being absent.
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- **created** *string*, OPTIONAL
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An combined date and time at which the image was created, formatted as defined by [RFC 3339, section 5.6][rfc3339-s5.6].
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- **author** *string*, OPTIONAL
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Gives the name and/or email address of the person or entity which created and is responsible for maintaining the image.
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- **architecture** *string*, REQUIRED
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The CPU architecture which the binaries in this image are built to run on.
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Configurations SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOARCH`][go-environment].
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- **os** *string*, REQUIRED
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The name of the operating system which the image is built to run on.
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Configurations SHOULD use, and implementations SHOULD understand, values listed in the Go Language document for [`GOOS`][go-environment].
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- **config** *object*, OPTIONAL
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The execution parameters which SHOULD be used as a base when running a container using the image.
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This field can be `null`, in which case any execution parameters should be specified at creation of the container.
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- **User** *string*, OPTIONAL
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The username or UID which is a platform-specific structure that allows specific control over which user the process run as.
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This acts as a default value to use when the value is not specified when creating a container.
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For Linux based systems, all of the following are valid: `user`, `uid`, `user:group`, `uid:gid`, `uid:group`, `user:gid`.
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If `group`/`gid` is not specified, the default group and supplementary groups of the given `user`/`uid` in `/etc/passwd` from the container are applied.
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- **ExposedPorts** *object*, OPTIONAL
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A set of ports to expose from a container running this image.
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Its keys can be in the format of:
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`port/tcp`, `port/udp`, `port` with the default protocol being `tcp` if not specified.
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These values act as defaults and are merged with any specified when creating a container.
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**NOTE:** This JSON structure value is unusual because it is a direct JSON serialization of the Go type `map[string]struct{}` and is represented in JSON as an object mapping its keys to an empty object.
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- **Env** *array of strings*, OPTIONAL
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Entries are in the format of `VARNAME=VARVALUE`.
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These values act as defaults and are merged with any specified when creating a container.
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- **Entrypoint** *array of strings*, OPTIONAL
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A list of arguments to use as the command to execute when the container starts.
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These values act as defaults and may be replaced by an entrypoint specified when creating a container.
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- **Cmd** *array of strings*, OPTIONAL
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Default arguments to the entrypoint of the container.
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These values act as defaults and may be replaced by any specified when creating a container.
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If an `Entrypoint` value is not specified, then the first entry of the `Cmd` array SHOULD be interpreted as the executable to run.
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- **Volumes** *object*, OPTIONAL
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A set of directories describing where the process is likely write data specific to a container instance.
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**NOTE:** This JSON structure value is unusual because it is a direct JSON serialization of the Go type `map[string]struct{}` and is represented in JSON as an object mapping its keys to an empty object.
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- **WorkingDir** *string*, OPTIONAL
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Sets the current working directory of the entrypoint process in the container.
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This value acts as a default and may be replaced by a working directory specified when creating a container.
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- **Labels** *object*, OPTIONAL
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The field contains arbitrary metadata for the container.
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This property MUST use the [annotation rules](annotations.md#rules).
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- **StopSignal** *string*, OPTIONAL
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The field contains the system call signal that will be sent to the container to exit. The signal can be a signal name in the format `SIGNAME`, for instance `SIGKILL` or `SIGRTMIN+3`.
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- **rootfs** *object*, REQUIRED
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The rootfs key references the layer content addresses used by the image.
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This makes the image config hash depend on the filesystem hash.
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- **type** *string*, REQUIRED
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MUST be set to `layers`.
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Implementations MUST generate an error if they encounter a unknown value while verifying or unpacking an image.
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- **diff_ids** *array of strings*, REQUIRED
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An array of layer content hashes (`DiffIDs`), in order from first to last.
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- **history** *array of objects*, OPTIONAL
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Describes the history of each layer.
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The array is ordered from first to last.
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The object has the following fields:
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- **created** *string*, OPTIONAL
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A combined date and time at which the layer was created, formatted as defined by [RFC 3339, section 5.6][rfc3339-s5.6].
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- **author** *string*, OPTIONAL
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The author of the build point.
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- **created_by** *string*, OPTIONAL
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The command which created the layer.
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- **comment** *string*, OPTIONAL
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A custom message set when creating the layer.
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- **empty_layer** *boolean*, OPTIONAL
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This field is used to mark if the history item created a filesystem diff.
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It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, Dockerfile's [ENV](https://docs.docker.com/engine/reference/builder/#/env) command results in no change to the filesystem).
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Any extra fields in the Image JSON struct are considered implementation specific and MUST be ignored by any implementations which are unable to interpret them.
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Whitespace is OPTIONAL and implementations MAY have compact JSON with no whitespace.
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## Example
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Here is an example image configuration JSON document:
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```json,title=Image%20JSON&mediatype=application/vnd.oci.image.config.v1%2Bjson
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{
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"created": "2015-10-31T22:22:56.015925234Z",
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"author": "Alyssa P. Hacker <alyspdev@example.com>",
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"architecture": "amd64",
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"os": "linux",
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"config": {
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"User": "alice",
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"ExposedPorts": {
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"8080/tcp": {}
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},
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"Env": [
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"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
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"FOO=oci_is_a",
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"BAR=well_written_spec"
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],
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"Entrypoint": [
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"/bin/my-app-binary"
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],
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"Cmd": [
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"--foreground",
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"--config",
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"/etc/my-app.d/default.cfg"
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],
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"Volumes": {
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"/var/job-result-data": {},
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"/var/log/my-app-logs": {}
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},
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"WorkingDir": "/home/alice",
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"Labels": {
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"com.example.project.git.url": "https://example.com/project.git",
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"com.example.project.git.commit": "45a939b2999782a3f005621a8d0f29aa387e1d6b"
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}
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},
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"rootfs": {
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"diff_ids": [
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"sha256:c6f988f4874bb0add23a778f753c65efe992244e148a1d2ec2a8b664fb66bbd1",
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"sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef"
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],
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"type": "layers"
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},
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"history": [
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{
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"created": "2015-10-31T22:22:54.690851953Z",
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"created_by": "/bin/sh -c #(nop) ADD file:a3bc1e842b69636f9df5256c49c5374fb4eef1e281fe3f282c65fb853ee171c5 in /"
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},
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{
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"created": "2015-10-31T22:22:55.613815829Z",
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"created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
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"empty_layer": true
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}
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]
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}
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```
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[rfc3339-s5.6]: https://tools.ietf.org/html/rfc3339#section-5.6
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[go-environment]: https://golang.org/doc/install/source#environment
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[tar-split]: https://github.com/vbatts/tar-split
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