fixup! ci(docker): run each platform on a different worker

.github/workflows/ci.yml

@@ -3,6 +3,7 @@ name: Proposal tests
 # run on all PRs
 on:
   pull_request:
+  workflow_dispatch:
   merge_group:
   push:
     branches: [main]
@@ -28,6 +29,7 @@ jobs:
           DEFAULT_PLATFORM=linux/amd64
           echo "default=$DEFAULT_PLATFORM" >> $GITHUB_OUTPUT
           if ${{ github.event_name == 'pull_request' || github.event_name == 'merge_group' }}; then
+            # JSON-encoded list consisting only of the default platform.
             platforms='["'"$DEFAULT_PLATFORM"'"]'
           else
             platforms='["linux/amd64","linux/arm64/v8"]'
@@ -105,11 +107,14 @@ jobs:
         id: docker-tags
         run: |
           sep=
+          # A list of comma-separated tags to merge in the final image.
           SUFFIXED=
+          # Our platform, replacing slashes with underscores.
           uarch=$(echo "${{ matrix.platform }}" | tr / _)
           for TAG in ${{ steps.meta.outputs.tags }}; do
             SUFFIXED="$SUFFIXED$sep$TAG-$uarch"
             if test -z "$sep"; then
+              # The first tag (suffixed with our architecture) is the one we build.
               sep=,
               echo "tag=$TAG-$uarch" >> $GITHUB_OUTPUT
             fi
@@ -169,6 +174,7 @@ jobs:
       - name: Push multiarch image
         run: |
           set -ex
+          # Push all tags, comprised of all architectures, to the registry.
           for TAG in ${{ steps.docker-tags.outputs.tags }}; do
             sources=
             for ARCH in ${{ join(fromJson(needs.platforms.outputs.platforms), ' ') }}; do
@@ -178,3 +184,12 @@ jobs:
             done
             docker buildx imagetools create --tag "$TAG"$sources
           done
+
+      - name: notify on failure
+        if: failure() && github.event_name != 'pull_request'
+        uses: ./.github/actions/notify-status
+        with:
+          webhook: ${{ secrets.SLACK_WEBHOOK_URL }}
+          from: ${{ secrets.NOTIFY_EMAIL_FROM }}
+          to: ${{ secrets.NOTIFY_EMAIL_TO }}
+          password: ${{ secrets.NOTIFY_EMAIL_PASSWORD }}