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

config/docker/Dockerfile.danbooru

@@ -61,7 +61,7 @@ RUN \
 
 FROM base as production
 
-ENV LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.2
+ENV LD_PRELOAD=libjemalloc.so.2
 ENV MALLOC_CONF=background_thread:true,narenas:2,dirty_decay_ms:1000,muzzy_decay_ms:0,tcache:false
 
 COPY --from=development /root /root

lib/tasks/danbooru.rake

@@ -104,11 +104,22 @@ namespace :danbooru do
   namespace :docker do
     # Usage: bin/rails danbooru:docker:build
     #
-    # Build a Docker image. Note that uncommited changes won't be included in
-    # the image; commit changes first before building the image.
-    desc "Build a Docker image based on latest commit"
+    # Build a Docker image. Note that uncommited changes won't be included in the image; commit changes to Git first before building the image.
+    desc "Build a Docker image"
     task :build do
-      system("git archive HEAD | docker buildx build - --build-arg SOURCE_COMMIT=$(git rev-parse HEAD) -t danbooru -f Dockerfile")
+      system("git archive HEAD | docker buildx build - --platform linux/amd64 --build-arg SOURCE_COMMIT=$(git rev-parse HEAD) --tag danbooru --tag danbooru:x86 --file Dockerfile --load")
+    end
+
+    # Usage: bin/rails danbooru:docker:build-arm
+    #
+    # Build a Docker image for ARM. You may need to install QEMU first if building on x86.
+    #
+    # * sudo apt-get install qemu binfmt-support qemu-user-static       # Install QEMU
+    # * docker run --rm -it --platform linux/arm64 ubuntu               # Test that QEMU works
+    # * docker run --rm -it --platform linux/arm64 danbooru:arm -- bash # Test that the Danbooru image works
+    desc "Build a Docker image for ARM"
+    task :"build-arm" do
+      system("git archive HEAD | docker buildx build - --platform linux/arm64 --build-arg SOURCE_COMMIT=$(git rev-parse HEAD) --tag danbooru --tag danbooru:arm --file Dockerfile --load")
     end
   end