Add proper tags to docker images

[ChristophB]

Changes

This pull request will generate additional docker image tags.

There are essentially two scenarios in which an image is created and published:

1. Pushing a commit to main branch will always add an additional tag containing a hash code.

2. Pushing a tag to main branch will add the semantic version number as image tag.

Previous image versions are retained.

See docker/metadata-action documentation for more information.

Some Notes

There is currently a check in place that prevents the "Build Docker Image" workflow from running, if there is no successful "tests" workflow (if: ${{ github.event.workflow_run.conclusion == 'success' }}). This will cause the workflow to be started and immediately skipped, rendering the workflow's on.push and on.pull_request conditions useless (see .github/workflows/docker-image.yml).

[ChristophB]

I updated the pull request to address the notes mentioned before.

Docker images are built by

• creating pull requests to main
• pushing commits to main
• creating releases
• manual dispatching

Only in case of releases and manual dispatches, the resulting images are persisted in the GitHub Docker Registry. Images built from releases will have a proper version tag, whereas manual dispatched images have the branch name and SHA as tag.