.github/workflows/build.yaml: YYYYMMDD -> YYYYMMDDRN tags

this will prevent cache invalidation issues in the future

.github/workflows/build.yaml

@@ -38,6 +38,20 @@ jobs:
       - name: Checkout
         uses: classabbyamp/treeless-checkout-action@v1
 
+      - name: Get image release
+        id: release
+        run: |
+          # gets the list of all date-shaped tags for the image, finds the most recent one
+          tag="$(skopeo list-tags "docker://ghcr.io/${{ github.repository_owner }}/void-${{ matrix.libc }}${{ matrix.variant }}" | \
+            jq -r '.Tags | sort | reverse | map(select(test("^[0-9]{8}(R[0-9]+)?$")))[0]')"
+          # tags from a different day or pre-YYYYMMDDRN
+          if [ "${tag%R*}" != "$(date -u +%Y%m%d)" ] || [ "${tag%R*}" = "${tag}" ]; then
+            rel=1
+          else
+            rel=$(( ${tag##*R} + 1 ))
+          fi
+          echo "rel=${rel}" >> "${GITHUB_OUTPUT}"
+
       - name: Docker metadata
         id: meta
         uses: docker/metadata-action@v4
@@ -47,7 +61,7 @@ jobs:
           tags: |
             type=sha,prefix=
             type=raw,value=latest,enable={{is_default_branch}}
-            type=raw,value={{date 'YYYYMMDD'}},enable={{is_default_branch}},priority=1000
+            type=raw,value={{date 'YYYYMMDD'}}R${{ steps.release.outputs.rel }},enable={{is_default_branch}},priority=1000
           flavor: latest=false
           labels: |
             org.opencontainers.image.authors=Void Linux team and contributors