Add concurrency to our workflows

[yannick-roeder]

Ideally the previous workflow is cancelled when the newer workflow is already running. This can be achieved with workflow concurrency. Adding this to every workflow should be sufficient:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}-docker
  cancel-in-progress: ${{ github.ref_name != github.event.repository.default_branch }}

Note that the suffix of the group is always specific to the workflow to avoid that different workflows cancel each other in the same run.