WIP: Allow for prebuilt docker image

[bschonec]

This pull request will allow for the operator to use his own Docker image which may be preconfigured with mock and mock environments.

Using a pre-configured Docker image decreases the run time by about two minutes; which can be a significant time saver when doing testing.

Let me know your thoughts. I see you have another PR #6 out there that seems to follow the same thinking. I've included a Dockerfile as well but I haven't yet made changes to the README.md to reflect how to build the image correctly (using the --privileged flag).

Of course, my changes in ci-simple.yml refer to MY branch. This is somewhat of a chicken-egg issue and will have to be changed to your project if you accept the changes.

[jw3]

Give #9 a try. Caching is integrated, after the first mock init you will see time savings until the spec is changed

      - uses: jw3/mock-srpm@cache2
        with:
          chroot: ...
          spec: ...
          cache: true

If you want to use a prebuilt image it exists

      - uses: jw3/mock-srpm@cache2
        with:
          chroot: ...
          spec: ...
          cache: true
          image: fedora:37

9 has some details in the description about when and how the caching happens. I still need to doc and test before merging.

This might be v1 when its done.

[jw3]

Appreciate the contrib though, but this needed woven into the caching so it happened all at once in #9.