### Fixed documentation for building docker image

I'm assuming that this was a mistake (but I could be missing something)
[here](https://github.com/infiniflow/ragflow/pull/4658/files#:~:text=docker%20build%20%2D%2Dbuild%2Darg%20LIGHTEN%3D1%20%2Df%20Dockerfile%20%2Dt%20infiniflow/ragflow%3Anightly%2Dslim%20.)
when the docker building instructions for the slim image was just
changed away from an actual `docker build` command to the mac os `docker
compose up` command. This is just reverting the change.

### Type of change

- [X] Documentation Update