chore(docker): attempt to automate docker releases

Author: benjie

.github/workflows/docker-hub.yaml

@@ -0,0 +1,44 @@
+name: Publish Docker image
+
+on:
+  push:
+    branches:
+      - v4
+    tags:
+      - 'v[0-9]+.[0-9]+.[0-9]+'
+
+jobs:
+  push_to_registry:
+    name: Push Docker image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+
+      - name: Login to Docker Hub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: graphile/postgraphile
+          flavor: |
+            latest=${{ github.ref == 'refs/heads/v4' }}
+          tags: |
+            type=raw,value=next
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}-{{minor}}
+            type=semver,pattern={{major}}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}