add build prod image workflow

.github/workflows/build-prod-image.yml

+name: Build PROD Image
+
+on:
+  release:
+    types:
+      - published
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Docker image tag'
+        required: true
+
+jobs:
+  build-prod-image:
+    name: Build PROD Image
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: papermerge/papermerge.js
+
+      - name: Build and push with MANUAL tag
+        uses: docker/build-push-action@v2
+        if: ${{ github.event.inputs.tag != '' }}
+        with:
+          push: true
+          tags: papermerge/papermerge.js:${{ github.event.inputs.tag }}
+          file: docker/Dockerfile
+
+      - name: Build and push with RELEASE version as tag
+        uses: docker/build-push-action@v2
+        if: ${{ github.event.inputs.tag == '' }}
+        with:
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          file: docker/Dockerfile
+          labels: ${{ steps.meta.outputs.labels }}