add auto generated dockerfiles

This MR adds a template for the docker files to hopefully reduce repetition in modifying the files. A single change to the template will propagate to the other files.

I also switched to the conda-forge version of multinest and ultranest. For the former this is a significant simplification.

Are people happy to make this change? Is it possible to remove all of the old docker files without breaking everything?

Also, I gave the files new names, rather than overwriting the existing ones, although I'm happy to go either way.

There may be a clever way to make the script run automatically when the template is changed, but I don't know how to do that.

@gregory.ashton @matthew-pitkin I know you've interacted with these files in the past.