README: Local development commands don't work

[razvand]

The README.md file features instructions for local development:

docker build -t ghcr.io/unikraft/docs:dev --target dev .

docker run -it --rm -v $(pwd):/docs -w /docs -p 3000:3000 ghcr.io/unikraft/docs:dev

However, when running these commands there is an error:

> @unikraft/docs@2.0.0 dev                                                                                                                                                                                                                                    
> next

sh: next: not found