fix: skip docs postinstall in Docker deps stage

[furtherref]

Summary

• skip package lifecycle scripts in the docs Docker deps stage so fumadocs-mdx does not run before source.config.ts is copied
• keep the full docs install/build flow in the builder stage after source files are available

Test plan

• git diff --check
• docker compose -f docker-compose.selfhost.yml -f docker-compose.selfhost.build.yml config --quiet
• STANDALONE=true pnpm --filter @multica/docs build
• pre-push hook: pnpm lint, pnpm typecheck, pnpm test, pnpm build

CI failure addressed

The merged docs Dockerfile ran pnpm install in the deps stage, which triggered apps/docs postinstall before apps/docs/source.config.ts existed in the image. This fixes the remote docker-docs-build failure.