feat(ci): build arm64 Docker images natively on ubuntu-24.04-arm

Split the Docker image build into a per-platform matrix and merge job,
producing a single multi-arch manifest (linux/amd64 + linux/arm64) without
QEMU emulation. The arm64 build runs on the native ubuntu-24.04-arm runner.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/docker-build-publish.yml

@@ -11,12 +11,15 @@ on:
         required: true
         type: string
 
+env:
+  IMAGE_NAME: ghcr.io/cbcoutinho/nextcloud-mcp-server
+
 jobs:
-  build-and-push:
+  resolve-tag:
     runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      packages: write
+    outputs:
+      tag: ${{ steps.resolve.outputs.tag }}
+      version: ${{ steps.resolve.outputs.version }}
     steps:
       - name: Resolve tag
         id: resolve
@@ -29,23 +32,37 @@ jobs:
           echo "tag=$TAG" >> $GITHUB_OUTPUT
           echo "version=${TAG#v}" >> $GITHUB_OUTPUT
 
+  build:
+    needs: resolve-tag
+    permissions:
+      contents: read
+      packages: write
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - platform: linux/amd64
+            runner: ubuntu-latest
+          - platform: linux/arm64
+            runner: ubuntu-24.04-arm
+    runs-on: ${{ matrix.runner }}
+    steps:
+      - name: Prepare platform pair
+        id: prep
+        run: |
+          platform="${{ matrix.platform }}"
+          echo "pair=${platform//\//-}" >> $GITHUB_OUTPUT
+
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
-          ref: ${{ steps.resolve.outputs.tag }}
+          ref: ${{ needs.resolve-tag.outputs.tag }}
 
       - name: Docker meta
         id: meta
         uses: docker/metadata-action@c299e40c65443455700f0fdfc63efafe5b349051 # v5.10.0
         with:
-          images: |
-            ghcr.io/cbcoutinho/nextcloud-mcp-server
-          tags: |
-            type=semver,pattern={{version}},value=${{ steps.resolve.outputs.tag }}
-            type=semver,pattern={{major}}.{{minor}},value=${{ steps.resolve.outputs.tag }}
-            type=semver,pattern={{major}},value=${{ steps.resolve.outputs.tag }}
-            type=sha
-            type=raw,value=latest
+          images: ${{ env.IMAGE_NAME }}
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
@@ -58,9 +75,73 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push Docker image
+      - name: Build and push by digest
+        id: build
         uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6.19.2
         with:
-          push: ${{ github.event_name != 'pull_request' }}
-          tags: ${{ steps.meta.outputs.tags }}
+          platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
+          outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest=true,name-canonical=true,push=${{ github.event_name != 'pull_request' }}
+
+      - name: Export digest
+        if: github.event_name != 'pull_request'
+        run: |
+          mkdir -p ${{ runner.temp }}/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "${{ runner.temp }}/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        if: github.event_name != 'pull_request'
+        uses: actions/upload-artifact@de65e23aa2b7e23d713bb51fbfcb6d502f8667d8 # v4.6.2
+        with:
+          name: digests-${{ steps.prep.outputs.pair }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    needs: [resolve-tag, build]
+    if: github.event_name != 'pull_request'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4.3.0
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3.7.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@c299e40c65443455700f0fdfc63efafe5b349051 # v5.10.0
+        with:
+          images: ${{ env.IMAGE_NAME }}
+          tags: |
+            type=semver,pattern={{version}},value=${{ needs.resolve-tag.outputs.tag }}
+            type=semver,pattern={{major}}.{{minor}},value=${{ needs.resolve-tag.outputs.tag }}
+            type=semver,pattern={{major}},value=${{ needs.resolve-tag.outputs.tag }}
+            type=sha
+            type=raw,value=latest
+
+      - name: Create manifest list and push
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          docker buildx imagetools create $(jq -cr '.tags | map("-t " + .) | join(" ")' <<< "$DOCKER_METADATA_OUTPUT_JSON") \
+            $(printf '${{ env.IMAGE_NAME }}@sha256:%s ' *)
+
+      - name: Inspect image
+        run: |
+          docker buildx imagetools inspect ${{ env.IMAGE_NAME }}:${{ steps.meta.outputs.version }}