offinfra-onboard: --dockerfile flag for non-root Dockerfiles

claude-memory-mcp's Dockerfile is at docker/Dockerfile, not repo root
(infra#20 build failed: 'open Dockerfile: no such file or directory').
build.yml template gains file: {{DOCKERFILE}} (default ./Dockerfile).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/offinfra-templates/build.yml.tmpl

@@ -63,6 +63,7 @@ jobs:
       - uses: docker/build-push-action@v7
         with:
           context: {{CONTEXT}}
+          file: {{DOCKERFILE}}
           push: true
           platforms: linux/amd64
           # Single-manifest images (no provenance/SBOM attestation children) so