Use a matrix build for building the docker image

Using parallel jobs means each worker can 100% dedicate itself to
building for the relevant platform, rather than splitting its
resources cross-compiling.

.github/workflows/docker-publish.yml

@@ -24,6 +24,11 @@ jobs:
   docker_build:
     #if: github.event_name != 'pull_request'
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        platform:
+          - linux/amd64
+          - linux/arm64
     permissions:
       contents: read
       packages: write
@@ -58,12 +63,12 @@ jobs:
 
       # Build and push Docker image with Buildx (don't push on PR)
       # https://github.com/docker/build-push-action
-      - name: Build and push Docker image
+      - name: Build and push Docker image for ${{matrix.platform}}
         id: build-and-push
         uses: docker/build-push-action@v4
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
+          platforms: ${{ matrix.platform }}
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}