Update metadata-action to 4 and adjust entrypoint

Author: white-gecko

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
       - name: Extract metadata (tags, labels) for Docker
         id: meta
-        uses: docker/metadata-action@v3.4.1
+        uses: docker/metadata-action@v4
         with:
           images: |
             ghcr.io/aksw/jekyll-rdf

docker-resources/entrypoint.sh

@@ -1,4 +1,9 @@
 #!/bin/sh
 
 bundle install
-jekyll build
+if [ $# -eq 0 ]
+  then
+    jekyll build
+  else
+    $@
+fi