Add GH Action to build the docker image

Builds the docker image on pushes for: tags, PRs, master.

Publishes docker images as per:
* `latest` & `<TAG>` for tags
* `master` for `master`

Same as https://github.com/matrix-org/lb/pull/8 & https://github.com/matrix-org/matrix-content-scanner/pull/46

.github/workflows/docker-publish.yml

@@ -0,0 +1,44 @@
+name: Container Image
+
+on:
+  push:
+    branches: [ master ]
+    tags: [ 'v*' ]
+  pull_request:
+    branches: [ master ]
+
+env:
+  IMAGE_NAME: ${{ github.repository }}
+  REGISTRY: ghcr.io
+
+jobs:
+  push:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Log into registry ${{ env.REGISTRY }}
+        uses: docker/login-action@v1
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v2
+        with:
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}