ci: publish immutable date-stamped image tags (#275)

Pass build-date-tag: true to the reusable docker-build workflow so that
master builds additionally publish an immutable <docker-tag>-<YYYYMMDD>
tag, mirroring the owncloud-docker/ubuntu base image repo.

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DeepDiver1975

.github/workflows/main.yml

@@ -24,6 +24,7 @@ jobs:
       docker-hub-username: ${{ vars.DOCKERHUB_USERNAME }}
       smoke-test-cmd: ${{ matrix.version.smoke-test-cmd }}
       push: ${{ github.ref == 'refs/heads/master' }}
+      build-date-tag: true
     secrets:
       docker-hub-password: ${{ secrets.DOCKERHUB_TOKEN }}
       docker-secrets: |