docker: add Docker image for ARM.

* Have CI build Docker images for both x86 and ARM.
* Add a `bin/rails danbooru:docker:build-arm` command for building a Docker image locally for ARM.

Usage:

* Test the image:

  docker run --rm -it --platform linux/arm64 ghcr.io/danbooru/danbooru bash

* Build the image:

  bin/rails danbooru:docker:build-arm

* Build the image by hand:

  git archive HEAD | docker buildx build - --platform linux/amd64 --build-arg SOURCE_COMMIT=$(git rev-parse HEAD) -t danbooru -f Dockerfile --load

.github/workflows/docker-build.yaml

@@ -27,6 +27,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      # https://github.com/docker/setup-qemu-action
+      # https://github.com/docker/setup-buildx-action#with-qemu
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+        with:
+          platforms: arm64
+
       # https://github.com/docker/setup-buildx-action
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
@@ -73,6 +80,8 @@ jobs:
           tags: ${{ steps.metadata.outputs.tags }}
           labels: ${{ steps.metadata.outputs.labels }}
 
+          platforms: linux/amd64,linux/arm64
+
           # https://github.com/docker/build-push-action/blob/master/docs/advanced/cache.md#registry-cache
           # https://dev.to/pst418/speed-up-multi-stage-docker-builds-in-ci-cd-with-buildkit-s-registry-cache-11gi
           cache-from: type=registry,ref=ghcr.io/${{ github.repository }}:buildcache