Fix ctsm-docs container version tagging

[samsrabin]

Description of changes

The GitHub workflow that builds and pushes new versions of the ctsm-docs container doesn't include a tag with the version number. This PR fixes that. It also checks that the included version tag is formatted correctly and hasn't been used before.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed:

• Resolves Increment docs container version #3126

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? Yes, the README.md file next to the Dockerfile; yes.

Testing performed, if any: Tested at https://github.com/samsrabin/test-CTSM-docs-version-tagging:

• Works if Dockerfile has exactly one new version
• Fails if version specified in Dockerfile already exists
• Fails if version is missing from Dockerfile
• Fails if two different versions are given in Dockerfile

[ekluzek]

@samsrabin and I went over this in depth. This is good work, and the changes make sense, and are needed for what happens.

This will be something that CTSM SE developers might need to work with on occasion and there's documentation on how to do the basic things. The whole package has complexity to it, but we likely won't need to modify it very often. I'm glad @samsrabin has developed expertise in this area, and we will undoubtably use that for other things as well.