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Document images.

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Eric Tune 2014-10-09 10:06:45 -07:00
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docs/container-environment.md
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@ -7,7 +7,7 @@ This document describes the environment for Kubelet managed containers on a Kube
This cluster information makes it possible to build applications that are *cluster aware*.  
Additionally, the Kubernetes container environment defines a series of signals that are surfaced to optional signal handlers defined as part of individual containers.  Container signals are somewhat analagous to operating system signals in a traditional process model.   However these signals are designed to make it easier to build reliable, scalable cloud applications in the Kubernetes cluster.  Containers that participate in this cluster lifecycle become *cluster native*. 
Another important part of the container environment is the file system that is available to the container. In Kubernetes, the filesystem is a combination of the Docker image and pod volumes. The design and usage of pod volumes is described in its own [document](docs/volumes.md)
Another important part of the container environment is the file system that is available to the container. In Kubernetes, the filesystem is a combination of an [image](./images.md) and one or more [volumes](./volumes.md).
The following sections describe both the cluster information provided to containers, as well as the signals and life-cycle that allows containers to interact with the management system.

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# Images
Each container in a pod has its own image. Currently, the only type of image supported is a [Docker Image](https://docs.docker.com/userguide/dockerimages/).
You create your Docker image and push it to a registry before referring to it in a kubernetes pod.
The `image` property of a container supports the same syntax as the `docker` command does, including private registries and tags.
## Using a Private Registry
Keys for private registries are stored in a `.dockercfg` file. Create a config file by running `docker login <registry>.<domain>` and then copying the resulting `.dockercfg` file to the kubelet working dir.
The kubelet working dir varies by cloud provider. It is `/` on GCE and `/home/core` on CoreOS. You can determine the working dir by running this command:
`sudo ls -ld /proc/$(pidof kubelet)/cwd` on a kNode.
All users of the cluster will have access to any private registry in the `.dockercfg`.
## Preloading Images
Be default, the kubelet will try to pull each image from the specified registry.
However, if the `imagePullPolicy` property of the container is set to `PullIfNotPresent` or `PullNever`,
then a local image is used (preferentially or exclusively, respectively).
This can be used to preload certain images for speed or as an alternative to authenticating to a private registry.
Pull Policy is per-container, but any user of the cluster will have access to all local images.