Push Docker image on workflow dispatch

[marip8]

Changes per commit message to allow for manual regeneration of Docker images.

[marip8]

Note, it appears that triggering a workflow_dispatch event looks to the action as if it came from the master branch, which causes the Docker meta-information step to generate a tag for type=ref,event=branch,prefix=${{ matrix.distro }}-,value=${{ env.BRANCH_NAME }}. This can be somewhat problematic when manually running the workflow at a git tag (e.g., 0.29.2) because it results in two Docker tags being created: <distro>-<major.minor> (as intended) and <distro>-master (which isn't truly isn't at the head of the master branch). Ultimately both created images get pushed to the registry.

I haven't come up with a good solution for this yet. The Docker meta-information tag specifications allow for enabling them with enable=..., so maybe there is something we can do there like:

type=ref,event=branch,prefix=${{ matrix.distro }}-,value=${{ env.BRANCH_NAME }},enable=${{ github.event_name != 'workflow_dispatch' }}

but the above specifically would prevent us from rebuilding and pushing the image for the master branch.

I also considered trying to extract the highest priority tag from the list of tags provided by ${{ steps.meta.outputs.tags }}, but I think the tags are provided as a CSV list, so ${{ fromJSON(steps.meta.outputs.tags)[0] }} does not work