Add CI to build base images

.github/workflows/buildx.yml

@@ -0,0 +1,97 @@
+name: Build & Push Docker image (GHCR)
+
+on:
+  push:
+    branches: [ "main" ]
+    tags: [ "v*", "release-*" ]
+  pull_request:
+    branches: [ "main" ]
+
+permissions:
+  contents: read
+  packages: write
+  attestations: write
+  id-token: write
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+  PLATFORMS: linux/arm64
+  DOCKERFILE: dockerfiles/Dockerfile.base
+  TARGET: archarm
+  TAG_SUFFIX: minimal-aarch64
+
+jobs:
+  build:
+    name: Build${{ github.event_name == 'pull_request' && ' (no push)' || '' }}
+    runs-on: ubuntu-latest
+
+    concurrency:
+      group: docker-${{ github.ref }}
+      cancel-in-progress: true
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU (for cross-compiling images)
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.CI_TOKEN }}
+
+      - name: Derive image name
+        id: img
+        run: |
+          echo "IMAGE=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}" >> $GITHUB_OUTPUT
+
+      - name: Extract Docker metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ steps.img.outputs.IMAGE }}
+          flavor: |
+            suffix=-${{ env.TAG_SUFFIX }},onlatest=true
+          # Tag strategy:
+          # - main branch -> :latest and :sha
+          # - tags like v1.2.3 -> :1.2.3 and :1.2 and :1
+          # - PRs -> :pr-<num>-<sha>
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,format=short,prefix=,suffix=,enable={{is_default_branch}}
+            type=ref,event=tag
+            type=ref,event=pr
+
+      - name: Build (and push when not a PR)
+        id: buildx
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ${{ env.DOCKERFILE }}
+          target: ${{ env.TARGET }}
+          platforms: ${{ env.PLATFORMS }}
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}      # mirrors: -t $(IMAGE):minimal-aarch64 (+ semver / sha variants)
+          labels: ${{ steps.meta.outputs.labels }}
+          # Your local command used --load; in CI we push instead. (--load is single-arch only and not suited for multi-arch CI.)
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          provenance: true
+          sbom: true
+
+      # Optional: attest the pushed image (supply chain)
+      - name: Attest image (SLSA-style)
+        if: github.event_name != 'pull_request'
+        uses: actions/attest-build-provenance@v1
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.buildx.outputs.digest }}
+          push-to-registry: true