Add GitHub action to build & push Docker image to GHCR

.github/workflows/docker-build-push.yaml

@@ -0,0 +1,32 @@
+name: Docker build & push image
+
+on:
+  push:
+    branches: [v4]
+  workflow_dispatch:
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set lowercase repository owner environment variable
+        run: |
+          echo "OWNER_LOWERCASE=${OWNER,,}" >>${GITHUB_ENV}
+        env:
+          OWNER: '${{ github.repository_owner }}'
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ env.OWNER_LOWERCASE }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          push: true
+          tags: ghcr.io/${{ env.OWNER_LOWERCASE }}/quartz:latest