add ability to extend rocker images with rocker

[tfoote]

rocker can generate images, for convenient sharing these images could be uploaded to a registry or the Dockerfile can be committed to a repository with auto builds enabled on dockerhub.

Then rocker could use that

• A --no-build option would be valuable to support this
• A way to output the Dockerfile to file would be useful.

Even more powerful would be if the extensions used in the layer were captured so that rocker run could generate the correct runtime arguments for the extensions embedded into the image. This could possibly be implemented using labels or marker files in the image. Potentially the image could embed the rocker extensions in a known location such that the extensions would not need to be installed on the host system.

This would be valuable for reproducible aggregate images. The host system verification checks could still be run.