Introduce Docker Build Validation Workflow

Author: FSG-Cat

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

@@ -0,0 +1,87 @@
+# SPDX-FileCopyrightText: 2026 Catalan Lover <catalanlover@protonmail.com>
+#
+# SPDX-License-Identifier: Apache-2.0
+# This file is derived from the ghcr-all-dev-branches.yml workflow, but instead of pushing its results, it only validates the Docker build. It also runs on all excluded branches to validate builds on excluded branches.
+
+name: "Validate Docker Build"
+
+# Main is not excluded due to that we want access to validate the build on main and to enable scout mode just like we have for breaking synapse changes.
+on:
+  push:
+    branches-ignore:
+      - "dependabot/**"
+      - "github-actions/**"
+  schedule:
+    - cron: "20 20 * * *"
+  merge_group:
+    branches: [main]
+
+concurrency:
+  group: docker-test-build-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  IMAGE_NAME: draupnir
+  PLATFORMS: linux/amd64,linux/arm64
+  IMG_SOURCE: https://github.com/${{ github.repository }}
+
+jobs:
+  validate-docker-build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Check out
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
+        with:
+          fetch-depth: 0
+          fetch-tags: true
+      - name: Set lowercase image owner
+        id: image_owner
+        run:
+          echo "image_owner=$(echo '${{ github.repository_owner }}' | tr
+          '[:upper:]' '[:lower:]')" >> $GITHUB_OUTPUT
+
+      # Needed for multi platform builds
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@ce360397dd3f832beb865e1373c09c0e9f86d70a
+        with:
+          platforms: ${{ env.PLATFORMS }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@4d04d5d9486b7bd6fa91e7baf45bbb4f8b9deedd
+
+      - name: Derive image tags
+        id: meta
+        uses: docker/metadata-action@030e881283bb7a6894de51c315a6bfe6a94e05cf
+        with:
+          images:
+            ghcr.io/${{ steps.image_owner.outputs.image_owner }}/${{
+            env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=sha,prefix=sha-
+
+      - name: Build image
+        id: push
+        uses: docker/build-push-action@bcafcacb16a39f128d818304e6c9c0c18556b85f
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: ${{ env.PLATFORMS }}
+          push: false
+          # Shared Buildx cache scope reused by all container image workflows.
+          # Keep the scope name aligned across workflows to maximize cache hits.
+          cache-from: type=gha,scope=draupnir-container-build
+          labels: |
+            org.opencontainers.image.source=${{ env.IMG_SOURCE }}
+            org.opencontainers.image.revision=${{ github.sha }}
+            org.opencontainers.image.version=${{ github.ref_name }}-${{ github.sha }}
+            org.opencontainers.image.ref.name=${{ github.ref_name }}
+            org.opencontainers.image.licenses=Apache-2.0
+
+          # prettier-ignore
+          outputs: type=image,name=target,annotation-index.org.opencontainers.image.description=Draupnir is a community management platform for Matrix.
+          sbom: true
+          provenance: true
+          tags: ${{ steps.meta.outputs.tags }}