Tidy up set up docs - particularly docker

[mrchrisadams]

We've had reports that the docker setup isn't quite working.

We'll need to fix the docker setup anyway, because our current use of Gitpod is likely to not be supported past April 2025.

These docs need to work:
https://greenweb.readthedocs.io/en/latest/working-with-docker.html

[mrchrisadams]

See also: #632

[ralokt]

I guess that's another thing I can add to the list of things fixed by #635 !

(edit: well, maybe not quite - but as far as I can tell, the steps should still be the same. I'll test it now.)

[ralokt]

OK, not 100%. As it is now with #635 , it's necessary to provide the environment to build the image.

Question is, should the docs change, or should the docker setup change - because arguably, there should be a startup script anyway. I outlined why I think so here: #637