ci: move image build off-infra to GHA -> ghcr (ADR-0002)

Generated by infra/scripts/offinfra-onboard: GHA builds+tests on the
GitHub mirror, pushes ghcr.io/viktorbarzin/claude-memory-mcp, then triggers the
Woodpecker deploy (repo 78). Old in-cluster build pipeline
removed:

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

.github/workflows/build.yml

@@ -70,6 +70,7 @@ jobs:
       - uses: docker/build-push-action@v7
         with:
           context: .
+          file: docker/Dockerfile
           push: true
           platforms: linux/amd64
           # Single-manifest images (no provenance/SBOM attestation children) so