build: use pnpm official Docker image and update CI/Docker setup (#639)

- Replace docker-compose.yml (node:lts) with compose.yaml (ghcr.io/pnpm/pnpm:11.3.0)
  - Bump pnpm to 11.3.0 and add --frozen-lockfile in CI
  - Add pnpm-workspace.yaml to allow esbuild and sharp builds
  - Update .gitignore for pnpm store and compose override file

.gitignore

@@ -24,4 +24,10 @@ pnpm-debug.log*
 .idea/
 
 # pagefind
-public/pagefind
+public/pagefind
+
+# pnpm
+.pnpm-store
+
+# docker compose
+compose.override.yaml