fix(gatekeeper): ensure public directory exists in Dockerfile to prevent COPY failure

packages/infra/docker/Dockerfile.gatekeeper

@@ -15,6 +15,7 @@ RUN --mount=type=cache,target=/root/.local/share/pnpm/store/v3 \
 
 # Build Gatekeeper and its dependencies
 RUN pnpm --filter @mintel/gatekeeper... build
+RUN mkdir -p packages/gatekeeper/public
 
 # Step 2: Runner stage
 FROM node:20-alpine AS runner