Prepare slim version of docker image containing spack release with running environment only

[dingp]

This can be achieved by flattening the docker image created by nightly workflow (or build-frozen release workflow).

The flattening process is like the following:

1. start up a container using the docker image containing the full Spack-release interactively;
2. prepare and run a script to trim down the release and externals, and even system packages;
3. commit the change to a new docker image;
4. flatter the image with docker export and docker import to remove all intermediate layers.

[dingp]

Deferred to post 3.0, we are using cvmfs mounted from host in 3.0.

[jcfreeman2]

This is still a potentially desirable thing; however right now implementing it isn't a priority. Please discuss with John, Andrew and Alessandro if this is something you want worked on.