perf: optimize gatekeeper docker build with cache mounts and layer caching

packages/infra/docker/Dockerfile.gatekeeper

@@ -4,15 +4,24 @@ RUN apk add --no-cache libc6-compat curl
 WORKDIR /app
 RUN corepack enable pnpm
 
-# Copy source (honoring .dockerignore)
-COPY . .
+# Copy manifest files specifically for better layer caching
+COPY pnpm-lock.yaml pnpm-workspace.yaml package.json .npmrc ./
+COPY packages/gatekeeper/package.json ./packages/gatekeeper/package.json
+COPY packages/next-utils/package.json ./packages/next-utils/package.json
+COPY packages/eslint-config/package.json ./packages/eslint-config/package.json
+COPY packages/next-config/package.json ./packages/next-config/package.json
+COPY packages/tsconfig/package.json ./packages/tsconfig/package.json
 
-# Use a secret for NPM_TOKEN to authenticate with private registry
-RUN --mount=type=cache,target=/root/.local/share/pnpm/store/v3 \
+# Use a secret for NPM_TOKEN and a cache mount for the pnpm store
+RUN --mount=type=cache,id=pnpm,target=/pnpm/store \
     --mount=type=secret,id=NPM_TOKEN \
     export NPM_TOKEN=$(cat /run/secrets/NPM_TOKEN) && \
+    pnpm config set store-dir /pnpm/store && \
     pnpm i --frozen-lockfile
 
+# Copy the rest of the source
+COPY . .
+
 # Build Gatekeeper and its dependencies
 RUN pnpm --filter @mintel/gatekeeper... build
 RUN mkdir -p packages/gatekeeper/public