WIP: Add CI job for multi-arch builds

.github/workflows/multi_arch_build.yml

@@ -0,0 +1,162 @@
+name: Multi-arch build
+
+on:
+  push:
+
+env:
+  REGISTRY: ghcr.io/${{ github.repository_owner }}
+  REGISTRY_USER: ${{ github.actor }}
+  REGISTRY_PASSWORD: ${{ github.token }}
+  IMAGE_NAME: dangerzone/dangerzone
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - linux/amd64
+          - linux/arm64
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Get current date
+        id: date
+        run: echo "date=$(date +'%Y%m%d')" >> $GITHUB_OUTPUT
+
+      - name: Prepare
+        run: |
+          platform=${{ matrix.platform }}
+          echo "PLATFORM_PAIR=${platform//\//-}" >> $GITHUB_ENV
+
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: |
+            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Login to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push by digest
+        id: build
+        uses: docker/build-push-action@v6
+        with:
+          context: ./dangerzone/
+          file: Dockerfile
+          build-args: |
+            DEBIAN_ARCHIVE_DATE=${{ steps.date.outputs.date }}
+          ## Remove potentially incorrect Docker provenance.
+          #provenance: false
+          platforms: ${{ matrix.platform }}
+          labels: ${{ steps.meta.outputs.labels }}
+          outputs: type=image,"name=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}",push-by-digest=true,name-canonical=true,push=true
+
+      - name: Export digest
+        run: |
+          mkdir -p ${{ runner.temp }}/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "${{ runner.temp }}/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v4
+        with:
+          name: digests-${{ env.PLATFORM_PAIR }}
+          path: ${{ runner.temp }}/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  merge:
+    runs-on: ubuntu-latest
+    needs:
+      - build
+    outputs:
+      digest: ${{ steps.image.outputs.digest }}
+      image: ${{ steps.image.outputs.image }}
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Compute image tag
+        id: tag
+        run: |
+          DATE=$(date +'%Y%m%d')
+          TAG=$(git describe --long --first-parent | tail -c +2)
+          echo "tag=${DATE}-${TAG}" >> $GITHUB_OUTPUT
+
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: ${{ runner.temp }}/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Login to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      #- name: Docker meta
+      #  id: meta
+      #  uses: docker/metadata-action@v5
+      #  with:
+      #    images: |
+      #      ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+      #    tags: |
+      #      type=ref,event=branch
+      #      type=ref,event=pr
+      #      type=semver,pattern={{version}}
+      #      type=semver,pattern={{major}}.{{minor}}
+
+      - name: Create manifest list and push
+        working-directory: ${{ runner.temp }}/digests
+        run: |
+          IMAGE=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.tag.outputs.tag }}
+          DIGESTS=$(printf '${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@sha256:%s ' *)
+          docker buildx imagetools create -t ${IMAGE} ${DIGESTS}
+
+      - name: Inspect image
+        id: image
+        run: |
+          # NOTE: Set the image as an output because the `env` context is not
+          # available to the inputs of a reusable workflow call.
+          image_name="${REGISTRY}/${IMAGE_NAME}"
+          echo "image=$image_name" >> "$GITHUB_OUTPUT"
+          docker buildx imagetools inspect ${image_name}:${{ steps.tag.outputs.tag }}
+          digest=$(docker buildx imagetools inspect ${image_name}:${{ steps.tag.outputs.tag }} --format "{{json .Manifest}}" | jq -r '.digest')
+          echo "digest=$digest" >> "$GITHUB_OUTPUT"
+
+  # This step calls the container workflow to generate provenance and push it to
+  # the container registry.
+  provenance:
+    needs:
+      - merge
+    permissions:
+      actions: read # for detecting the Github Actions environment.
+      id-token: write # for creating OIDC tokens for signing.
+      packages: write # for uploading attestations.
+    uses: slsa-framework/slsa-github-generator/.github/workflows/generator_container_slsa3.yml@v2.0.0
+    with:
+      digest: ${{ needs.merge.outputs.digest }}
+      image: ${{ needs.merge.outputs.image }}
+      registry-username: ${{ github.actor }}
+    secrets:
+      registry-password: ${{ secrets.GITHUB_TOKEN }}