Build and push multi-platform docker image (#1567)

This PR adds the `publish-docker-image-multiplatform` CI job step that
pushes a multi-platform docker image when a new tag is created. #1211

Unfortunately, this is not as simple as pushing two images targeting
different platforms to the same tag. There are a couple of ways to
create a multi-platform image:
- Using `docker buildx` to build images for all platforms. To do this
images targeting non-native platforms would need to be built using QEMU
(slower), a remote machine or cross-compilation.
- Building and publishing platform-specific images
`emscripten/emsdk-x64`, `emscripten/emsdk-arm64` and then creating a
manifest that links to these images. This is the simpler solution to
implement, and is what this PR does.

Tested with CircleCI on my fork. See the pushed images at
[radiantly/emsdk ·
DockerHub](https://hub.docker.com/r/radiantly/emsdk/tags)

.circleci/config.yml

@@ -96,7 +96,6 @@ jobs:
             python3 -m pip install flake8==7.1.1
             python3 -m flake8 --show-source --statistics --extend-exclude=./scripts
 
-
   test-linux:
     executor: ubuntu
     environment:
@@ -249,7 +248,7 @@ jobs:
           name: push image
           command: |
             docker login -u "$DOCKER_USER" -p "$DOCKER_PASS"
-            make -C ./docker version=${CIRCLE_TAG} alias=latest push
+            make -C ./docker version=${CIRCLE_TAG} alias=${CIRCLE_TAG}-x64 only_alias=true push
 
   publish-docker-image-arm64:
     executor: linux_arm64
@@ -267,6 +266,16 @@ jobs:
             docker login -u "$DOCKER_USER" -p "$DOCKER_PASS"
             make -C ./docker version=${CIRCLE_TAG} alias=${CIRCLE_TAG}-arm64 only_alias=true push
 
+  publish-docker-image-multiplatform:
+    executor: linux_arm64
+    steps:
+      - checkout
+      - run:
+          name: push image
+          command: |
+            docker login -u "$DOCKER_USER" -p "$DOCKER_PASS"
+            make -C ./docker version=${CIRCLE_TAG} alias="latest" push-multiplatform
+
   test-bazel7-linux:
     executor: ubuntu
     environment:
@@ -345,6 +354,14 @@ workflows:
               ignore: /.*/
             tags:
               only: /.*/
+      - publish-docker-image-multiplatform:
+          filters:
+            tags:
+              only: /.*/
+          requires:
+            - publish-docker-image-x64
+            - publish-docker-image-arm64
+
   test-bazel7-linux:
     jobs:
       - test-bazel7-linux

docker/Makefile

@@ -29,3 +29,7 @@ ifdef alias
 	docker tag ${image_name}:${version} ${image_name}:${alias}
 	docker push ${image_name}:${alias}
 endif
+
+push-multiplatform: .TEST
+	docker manifest create ${image_name}:${version} $(foreach platform,x64 arm64,--amend ${image_name}:${version}-$(platform))
+	docker manifest push ${image_name}:${version}