Add a GitHub Action CI check to ensure Dockerfile changes actually wo…

…rk (#248)

.github/workflows/docker_check.yml

@@ -0,0 +1,48 @@
+# GitHub actions workflow which builds the docker images.
+# This is useful as it will run in PRs that change the Dockerfile, and can be
+# an early source of warnings that the Dockerfile isn't right.
+# This check also triggers when this file itself is modified.
+
+name: Check Docker image can be built successfully
+
+on:
+  push:
+    paths:
+      - 'docker/Dockerfile'
+      - '.github/workflows/docker_check.yml'
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up QEMU
+        id: qemu
+        uses: docker/setup-qemu-action@v1
+        with:
+          platforms: arm64
+
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Inspect builder
+        run: docker buildx inspect
+
+      # we explicitly check out the repository (and use `context: .` in buildx)
+      # because we need to preserve the git metadata so that setuptools_scm
+      # (as part of Sygnal's setup.py) can deduce the package version.
+      # See: https://github.com/marketplace/actions/build-and-push-docker-images#path-context
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Build all platforms
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: false
+          labels: "gitsha1=${{ github.sha }}"
+          file: "docker/Dockerfile"
+          platforms: linux/amd64,linux/arm64

changelog.d/248.docker

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+CI now checks that the Docker image still builds after the Dockerfile is modified.