moby: CMD doesn't survive docker run, docker commit

When an image that has a CMD is run, and the resulting container committed as a new image, the new image has no CMD.

As a counterpoint, when an image that has a CMD is used as the FROM line in a Dockerfile, the new image DOES have a CMD. (Perhaps subject to bug(s) vaguely described in #3762).

I think it should be default, or at least possible, for the resulting image to retain the CMD / config.Cmd.

Reproduce

Here’s a minimal reproduction:

#!/bin/bash
set -e

# Create image with explicit CMD in Dockerfile
cat > Dockerfile <<END
FROM ubuntu:13.10
CMD ["true"]
END
docker build --rm --tag cmdtest:dockerfile . > /dev/null 2>&1

# Run and commit a container as a new image.
rm -f container_id
docker run --cidfile=container_id cmdtest:dockerfile
docker commit $(cat container_id) cmdtest:dockerrun > /dev/null

# Output: Inspect CMD
set -x
docker inspect --format='{{.config.Cmd}}' cmdtest:dockerfile
docker inspect --format='{{.config.Cmd}}' cmdtest:dockerrun
docker run cmdtest:dockerrun

Output:

+ docker inspect '--format={{.config.Cmd}}' cmdtest:dockerfile
[true]
+ docker inspect '--format={{.config.Cmd}}' cmdtest:dockerrun
<no value>
+ docker run cmdtest:dockerrun
2014/02/26 23:52:00 Error: create: No command specified

How it works in Dockerfile

The Dockerfile CMD line does this in buildfile.go (error handling trimmed):

func (b *buildFile) CmdCmd(args string) error {
    cmd := b.buildCmdFromJson(args)
    b.config.Cmd = cmd
    _ := b.commit("", b.config.Cmd, fmt.Sprintf("CMD %v", cmd))
}

I can’t find a docker command to manipulate an image config to reproduce b.config.Cmd = cmd outside of a Dockerfile. Nothing else inside Docker seems to write to config.Cmd.

Use-case

My use-case is a build pipeline which takes a developer-friendly image (codebase is mounted in as a volume), and builds a deployable image by adding and building the codebase using something roughly like this:

docker run \
  --cidfile="build.cid"
  --volume=/codebase:/mnt/codebase \
  IMAGE_NAME \
  bash -c "cp -a /mnt/codebase /codebase && cd /codebase && make"

docker commit $(cat build.cid) release

I’d like the release image to retain the CMD from the original image.

Version

$ docker -v
Docker version 0.8.1, build a1598d1

$ docker info
Containers: 25
Images: 68
Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Dirs: 118

$ uname -a
Linux ubuntu-13 3.11.0-17-generic #31-Ubuntu SMP Mon Feb 3 21:52:43 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux