refactor: restructure Dockerfile.gatekeeper into base, builder, and runner stages for improved multi-stage build.

packages/infra/docker/Dockerfile.gatekeeper

@@ -1,14 +1,11 @@
-FROM node:20-alpine AS builder
-
+FROM node:20-alpine AS base
 RUN apk add --no-cache libc6-compat curl
 WORKDIR /app
-
-# Enable pnpm
 RUN corepack enable pnpm
 
-# Step 2: Install dependencies
-# We copy everything first because we have a .dockerignore
-# and we need the workspace structure for pnpm to work correctly
+# Step 2: Builder stage
+FROM base AS builder
+# Copy source (honoring .dockerignore)
 COPY . .
 
 # Use a secret for NPM_TOKEN to authenticate with private registry
@@ -17,13 +14,10 @@ RUN --mount=type=cache,target=/root/.local/share/pnpm/store/v3 \
     export NPM_TOKEN=$(cat /run/secrets/NPM_TOKEN) && \
     pnpm i --frozen-lockfile
 
-# Copy source
-COPY . .
-
 # Build Gatekeeper
 RUN pnpm --filter @mintel/gatekeeper build
 
-# Runner
+# Step 3: Runner stage
 FROM base AS runner
 WORKDIR /app
 ENV NODE_ENV=production