feat(ci): run on real arm64 builders

Emulated arm64 builds are slow. Fortunately, GitHub has real Linux ARM64
hardware these days, so let's switch to it.

.github/workflows/docker.yml

@@ -29,13 +29,14 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
         platform:
-          - linux/amd64
-          - linux/arm64
+          - ubuntu-latest
+          - github-hosted-ubuntu-arm64
+
+    runs-on: ${{ matrix.platform }}
 
     steps:
       - name: Checkout
@@ -54,29 +55,28 @@ jobs:
         uses: docker/setup-buildx-action@988b5a0280414f521da01fcc63a27aeeb4b104db # v3.6.1
 
       - name: Login to DockerHub
-        if: github.event_name == 'push'
+        if: github.event_name != 'pull_request'
         uses: grafana/shared-workflows/actions/dockerhub-login@main
 
       - name: Build and push by digest
         id: build
         uses: docker/build-push-action@32945a339266b759abcbdc89316275140b0fc960 # v6.8.0
         with:
           context: .
-          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
-          outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=${{ github.event_name == 'push' }}
+          outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=${{ github.event_name != 'pull_request' }}
 
       - name: Export digest
         id: digest
-        if: github.event_name == 'push'
+        if: github.event_name != 'pull_request'
         run: |
           mkdir -p /tmp/digests
           digest="${{ steps.build.outputs.digest }}"
           touch "/tmp/digests/${digest#sha256:}"
           echo "artifact_name=digests-${{ matrix.platform }}" | sed -e 's/\//-/g' >> "$GITHUB_OUTPUT"
 
       - name: Upload digest
-        if: github.event_name == 'push'
+        if: github.event_name != 'pull_request'
         uses: actions/upload-artifact@50769540e7f4bd5e21e526ee35c689e35e0d6874 # v4.4.0
         with:
           name: ${{ steps.digest.outputs.artifact_name }}
@@ -86,20 +86,20 @@ jobs:
 
   merge:
     runs-on: ubuntu-latest
-    if: github.event_name == 'push'
+    if: github.event_name != 'pull_request'
     needs:
       - build
     steps:
       - name: Download digests (linux/amd64)
         uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
         with:
-          name: digests-linux-amd64
+          name: digests-github-hosted-ubuntu-arm64
           path: /tmp/digests-linux-amd64
 
       - name: Download digests (linux/arm64)
         uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
         with:
-          name: digests-linux-arm64
+          name: digests-ubuntu-latest
           path: /tmp/digests-linux-arm64
 
       - name: Merge digests