The `mount-tester` image is currently used as a base layer for other
test images (like `mounttest-user`) which perform uid/gid changes.
However, the binary built in here just follows local umask, and currently is
```
-rwxr-x--- 1 root root 2052704 May 19 2016 mt
```
This commit adds an explicit chmod on the binary to make sure uid/gid
tests can still run it as "others".
We can then avoid the following warning:
```
WARNING: The '--' argument must be specified between gcloud specific args on the left and DOCKER_ARGS on the right. IMPORTANT: previously, commands allowed the omission of the --, and unparsed arguments were treated as implementation args. This usage is being deprecated and will be removed in March 2017.
This will be strictly enforced in March 2017. Use 'gcloud beta docker' to see new behavior.
```
Signed-off-by: Jess Frazelle <acidburn@google.com>