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

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