Update to docker-push-action v2 w/ metadata, buildx (#228)

Docker Hub no longer builds for us, so we need to switch to GHA

.github/workflows/docker-build.yml

@@ -7,9 +7,32 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: docker/build-push-action@v1
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v3
         with:
-          repository: inbucket/inbucket
-          push: false
-          tag_with_ref: true
+          images: |
+            inbucket/inbucket
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=sha
+            type=edge,branch=develop
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to DockerHub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}