github: re-add rolling 'daily-testing-only' tag [skip ci]

This commit re-adds the rolling `daily-testing-only` tag in addition to
the unique daily tag `daily-<YYYYmmdd>`.

A rolling `master` build was proposed but declined because this would
put an unnecessary burden on the build system (each run takes ~45 min)

.github/workflows/docker.yml

@@ -41,7 +41,7 @@ jobs:
         if: github.event.schedule == '0 0 * * *'
         run: |
           echo "RELEASE_VERSION=master" >> $GITHUB_ENV
-          echo "IMAGE_TAG=daily-testing-$(date -u +%Y%m%d)" >> $GITHUB_ENV
+          echo "IMAGE_TAG=daily-testing-$(date -u +%Y%m%d),${DOCKER_REPO}/${DOCKER_IMAGE}:daily-testing-only" >> $GITHUB_ENV
 
       - name: Build and push
         id: docker_build