build(docker): improve docker build and push workflow

.github/workflows/docker.yml

@@ -61,6 +61,7 @@ jobs:
       # Extract metadata (tags, labels) for Docker
       # https://github.com/docker/metadata-action
       - name: Extract Docker metadata
+        if: github.repository == 'cshum/imagor'
         id: meta
         uses: docker/metadata-action@v4
         with:
@@ -73,6 +74,18 @@ jobs:
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
 
+      - name: Extract Docker metadata on fork
+        if: github.repository != 'cshum/imagor'
+        id: meta-fork
+        uses: docker/metadata-action@v4
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
       # Build and push Docker image with Buildx (don't push on PR)
       # https://github.com/docker/build-push-action
       - name: Build and push Docker image on semver