perf(docker): Add BuildKit cache mounts for faster builds

Added cache mounts for:
- pnpm store: Caches downloaded packages between builds
- TurboRepo: Caches build outputs between builds

This significantly speeds up subsequent builds:
- First build: Full download and compile
- Subsequent builds: Only changed packages are re-downloaded/rebuilt

Requires Docker BuildKit (default in Docker 23+).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/Dockerfile

@@ -1,3 +1,6 @@
+# syntax=docker/dockerfile:1
+# Enable BuildKit features for cache mounts
+
 # Base image for all stages
 FROM node:20-alpine AS base
 
@@ -22,8 +25,9 @@ COPY packages/ui/package.json ./packages/ui/
 COPY packages/config/package.json ./packages/config/
 COPY apps/api/package.json ./apps/api/
 
-# Install dependencies
-RUN pnpm install --frozen-lockfile
+# Install dependencies with pnpm store cache
+RUN --mount=type=cache,id=pnpm-store,target=/root/.local/share/pnpm/store \
+    pnpm install --frozen-lockfile
 
 # ======================
 # Builder stage
@@ -41,7 +45,9 @@ COPY apps/api ./apps/api
 
 # Build the API app and its dependencies using TurboRepo
 # This ensures @mosaic/shared is built first, then prisma:generate, then the API
-RUN pnpm turbo build --filter=@mosaic/api
+# Cache TurboRepo build outputs for faster subsequent builds
+RUN --mount=type=cache,id=turbo-cache,target=/app/.turbo \
+    pnpm turbo build --filter=@mosaic/api
 
 # ======================
 # Production stage