github: Build the multiplatform docker image in two steps

6b6b1240e2 tried to build a
multiplatform image, by building two images separately and pushing
them one at a time, with the same name (but with a different platform).

This doesn't work as expected; the last push for the image name
replaces earlier pushes. So depending on which push finishes
last, we'd end up with either an amd64 or arm64 image, but not
one that supports both.

Instead, push two separate images just with push-by-digest, and
then create the actual tag with "docker buildx imagetools create".

Alternatively, we could build all platform variants of the main
image with Dockerfile.toolchain, simplifying the setup significantly.
The contents of such a build is marginally different though, so
keep the amd64 image as it was, for now, compiled in that environment.

.github/workflows/docker.yml

@@ -38,15 +38,9 @@ jobs:
           cat parameters.txt >> $GITHUB_OUTPUT
           cat $GITHUB_OUTPUT
 
-  docker-build:
+  docker-build-dev:
     needs: [prepare]
     runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - { file: Dockerfile,     maintag: latest,  prefix: }
-          - { file: Dockerfile.dev, maintag: dev,     prefix: dev- }
     steps:
       - uses: actions/checkout@v4
         with:
@@ -62,22 +56,29 @@ jobs:
         with:
           context: .
           push: ${{inputs.push}}
-          file: ./${{matrix.file}}
+          file: ./Dockerfile.dev
           tags: |
-            mstorsjo/llvm-mingw:${{matrix.maintag}}
-            mstorsjo/llvm-mingw:${{matrix.prefix}}${{needs.prepare.outputs.TAG}}
+            mstorsjo/llvm-mingw:dev
+            mstorsjo/llvm-mingw:dev-${{needs.prepare.outputs.TAG}}
       - name: Inspect Docker images
         run: |
           docker images
 
-  docker-build-toolchain:
+  docker-build:
     needs: [prepare]
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { file: Dockerfile, key: amd64 }
+          - { file: Dockerfile.toolchain, platforms: linux/arm64, key: arm64 }
     steps:
       - uses: actions/checkout@v4
         with:
           ref: ${{ (inputs.commit != '' && inputs.commit) || inputs.branch }}
       - name: Download toolchain
+        if: ${{matrix.file == 'Dockerfile.toolchain'}}
         uses: dawidd6/action-download-artifact@v3
         with:
           workflow: build.yml
@@ -89,6 +90,7 @@ jobs:
           name_is_regexp: true
           path: toolchain
       - name: Set up QEMU
+        if: ${{matrix.file == 'Dockerfile.toolchain'}}
         uses: docker/setup-qemu-action@v3
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -100,15 +102,42 @@ jobs:
           password: ${{secrets.DOCKER_PASSWORD}}
       - name: Build Docker images
         uses: docker/build-push-action@v5
+        id: build
         with:
           context: .
-          platforms: linux/arm64
+          platforms: ${{matrix.platforms}}
           push: ${{inputs.push}}
-          file: ./Dockerfile.toolchain
-          load: ${{!inputs.push}}
-          tags: |
-            mstorsjo/llvm-mingw:latest
-            mstorsjo/llvm-mingw:${{needs.prepare.outputs.TAG}}
-      - name: Inspect Docker images
+          file: ./${{matrix.file}}
+          outputs: |
+            type=image,name=mstorsjo/llvm-mingw,push-by-digest=true,name-canonical=true
+      - name: Write outputs for later steps
+        uses: cloudposse/github-action-matrix-outputs-write@main
+        id: out
+        with:
+          matrix-step-name: ${{github.job}}
+          matrix-key: ${{matrix.key}}
+          outputs: |-
+            digest: ${{steps.build.outputs.digest}}
+
+  docker-create:
+    needs: [docker-build, prepare]
+    runs-on: ubuntu-latest
+    if: ${{inputs.push}}
+    steps:
+      - uses: cloudposse/github-action-matrix-outputs-read@main
+        id: read
+        with:
+          matrix-step-name: docker-build
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{secrets.DOCKER_USERNAME}}
+          password: ${{secrets.DOCKER_PASSWORD}}
+      - name: Create final image
         run: |
-          docker images
+          set -x
+          for tag in latest ${{needs.prepare.outputs.TAG}}; do
+              docker buildx imagetools create -t mstorsjo/llvm-mingw:$tag mstorsjo/llvm-mingw@${{fromJson(steps.read.outputs.result).digest.amd64}} mstorsjo/llvm-mingw@${{fromJson(steps.read.outputs.result).digest.arm64}}
+          done