fix(docker): Use TurboRepo to build workspace dependencies

The Docker builds were failing because they ran `pnpm build` directly
in the app directories without first building workspace dependencies
(@mosaic/shared, @mosaic/ui). CI passed because it runs TurboRepo
from the root which respects the dependency graph.

Changed both Dockerfiles to use `pnpm turbo build --filter=@mosaic/{app}`
which ensures dependencies are built in the correct order:
- Web: @mosaic/config → @mosaic/shared → @mosaic/ui → @mosaic/web
- API: @mosaic/config → @mosaic/shared → prisma:generate → @mosaic/api

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

apps/web/Dockerfile

@@ -39,15 +39,13 @@ COPY --from=deps /app/apps/web/node_modules ./apps/web/node_modules
 COPY packages ./packages
 COPY apps/web ./apps/web
 
-# Set working directory to web app
-WORKDIR /app/apps/web
-
 # Build arguments for Next.js
 ARG NEXT_PUBLIC_API_URL
 ENV NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL}
 
-# Build the application
-RUN pnpm build
+# Build the web app and its dependencies using TurboRepo
+# This ensures @mosaic/shared and @mosaic/ui are built first
+RUN pnpm turbo build --filter=@mosaic/web
 
 # ======================
 # Production stage