Add date-based docker image tag

[hbruch]

This PR introduces date-based image tags (Y-%m-%dT%H-%M) for docker images built on the main branch.

[derhuerst]

The changes look good, but I have some minor remarks.

Instead of the modifying the environment, you could also use GitHub's output feature if you want, which is a bit more verbose:

    - name: determine date-time-based tag
      id: docker-img-tag
      run: |
        echo "::set-output name=tag::$(date +%Y-%m-%dT%H-%M)"

          tags: |
            ghcr.io/${{github.repository}}:latest
            ghcr.io/${{github.repository}}:${{steps.docker-img-tag.outputs.tag}}

[derhuerst]

Given that the tag contains hours and minutes, let's call the variable DATE_TIME_TAG.

Although I couldn't provoke a collision with Bash-builtin syntax, I'd say it's better to be on the safe side and put the format string into single quotes.

Suggested change

	echo "DATE_TAG=$(date +%Y-%m-%dT%H-%M)" >> $GITHUB_ENV
	echo "DATE_TIME_TAG=$(date '+%Y-%m-%dT%H-%M')" >> $GITHUB_ENV

[derhuerst]

Suggested change

	ghcr.io/${{ github.repository }}:${{ env.DATE_TAG }}
	ghcr.io/${{ github.repository }}:${{ env.DATE_TIME_TAG }}