[ci] rewire docker image metadata

this should now also work if the repo name isn't all lowercase

.github/workflows/ci.yml

@@ -3,6 +3,10 @@ name: CI
 on:
   push:
 
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
 jobs:
   maven:
     runs-on: ubuntu-latest
@@ -38,9 +42,15 @@ jobs:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
-
+      # https://github.com/docker/metadata-action (lower cases image name, etc.)
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@98669ae865ea3cffbcbaa878cf57c20bbf1c6c38
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
       - name: Build and push
         uses: docker/build-push-action@v2
         with:
-          push: true
-          tags: ghcr.io/${{ github.repository }}:${{ github.ref_name }}
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}