Save "COMMENT" field in Dockerfile into image content.

This will allow a user to save COMMENT data into an image, which
can later be retrieved using:

docker inspect IMAGEID
your adding of `commit --message` makes me wonder - will `COMMENT` be set on all images created by the build, only the images after the COMMENT instruction, or only the final one (need to put the answer in the doc)

If I have more than one `COMMENT` in the Dockerfile, will it be like `WORKDIR` and be in effect from that point on, or like `ENTRYPOINT` and only the last applies.