fix(with-docker): resolve build with pnpm (#61529)
### Adding or Updating Examples
- Example:
[with-docker](https://github.com/vercel/next.js/tree/canary/examples/with-docker)
- Update: The latest change introduced in #61466 on the building process
of NextJS when the package manager is PNPM is currently throwing an
error (it works for npm though).
**To replicate the error:**
1. Checkout on the **with-docker** example
2. Install with PNPM so that it generates the pnpm-lock.yaml
3. Build the Docker image -> This will fail on the _builder stage_ step
that runs `pnpm run build`.
**Possible Solution:**
- Run `corepack enable pnpm` before building with pnpm
