Use ctsm-docs container

[samsrabin]

Instead of using the huge escomp/base Docker container, use a smaller ctsm-docs container that also contains some additional Sphinx functionality.

Blocks ESCOMP/CTSM#2809.

Remaining work:

• After pushing that container to GitHub, change DOCKER_IMAGE = line in build_commands.py to match the container name.
• Tag it to use a specific hash or tag instead of latest