Fix Docker workflow: lint Dockerfiles instead of building

The Dockerfiles are templates meant to be built from their respective
service repos, not from within the infra repo. Switch to hadolint
for Dockerfile linting and docker compose config for validation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker-build.yml

@@ -1,5 +1,5 @@
 # Copyright (c) 2025-2026 BlackRoad OS, Inc. All Rights Reserved.
-name: Docker Build
+name: Docker Lint
 
 on:
   push:
@@ -10,18 +10,20 @@ on:
     paths: ['docker/**']
 
 jobs:
-  build:
+  lint:
     runs-on: ubuntu-latest
     strategy:
       matrix:
         service: [core, web, agents, operator]
     steps:
       - uses: actions/checkout@v4
-      - uses: docker/setup-buildx-action@v3
+      - uses: hadolint/hadolint-action@v3.1.0
-      - uses: docker/build-push-action@v6
         with:
-          context: docker/${{ matrix.service }}
+          dockerfile: docker/${{ matrix.service }}/Dockerfile
-          push: false
+
-          tags: blackroad/${{ matrix.service }}:${{ github.sha }}
+  compose:
-          cache-from: type=gha
+    runs-on: ubuntu-latest
-          cache-to: type=gha,mode=max
+    steps:
+      - uses: actions/checkout@v4
+      - name: Validate docker-compose
+        run: docker compose -f docker/docker-compose.yml config --quiet