Regularly rebuild image of latest release

[andyundso]

Fixes FCREPO-3849 and FCREPO-3850.

This PR adds a new GitHub Actions workflow. In a nutshell:

1. It finds the latest Git tag on the main branch for fcrepo/fcrepo, excluding tags that include RC. For the found tag, we can safely assume it's the latest release of fcrepo.
2. Checks out the tag and rebuilds fcrepo.
3. Builds the Docker image and pushes three tags, as of now:
   • 6-tomcat9
   • 6.4-tomcat9
   • 6.4.0-tomcat9

The pipeline is triggered:

• when requested using workflow_dispatch.
• every Monday.
• on push to main.

A successful run looks something like this.

Note that this pipeline could be simplified:

1. Merge this repository with the main fcrepo, so we don't have to checkout two repository to make this pipeline work.
2. Extend the Dockerfile to also include the maven build, so this pipeline's instructions can be simplified.

[Surfrdan]

Looks good to me and targets same JDK versions as fcrepo/fcrepo

[tomcbe]

Some small questions/comments, but overall the changes look good.

Thanks @andyundso and sorry for the long delay.

[tomcbe]

Note that this pipeline could be simplified:

1. Merge this repository with the main `fcrepo`, so we don't have to checkout two repository to make this pipeline work.

2. Extend the Dockerfile to also include the `maven build`, so this pipeline's instructions can be simplified.

I agree, that the pipeline could be greatly simpilified. When the Docker image for Fedora 6 was created initially, some in the Fedora community had reservations against integrating the Docker image into the main fcrepo repository.

We can check if that's still the case, once this PR has been merged.

When sticking with the 2-repository solution, I think the way forward is:

• Only download the fcrepo-war from the maven repository (no need to rebuild it with maven)
• unpack and add it to the correct location in the Docker image.

[tomcbe]

The old variable name a is still to construct the tag with the full version number.

Should be:

"${VERSION_PARTS[0]}.${VERSION_PARTS[1].${VERSION_PARTS[2]}-tomcat9"

[tomcbe]

PR looks good now. I also tested the commands locally and was able to correctly build the Docker image.

[tomcbe]

@Surfrdan Can you merge this? I'm a bit unsure what procedure there is to merge PRs in this repo.