Make a nf training docker image

[chriswyatt1]

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Not essential, but it would be best if we have all the programs loaded as a single image.

At the moment, we have all the code being run in the .gitpod.yml init and script sections.

I assume it will be faster and simpler to have this prebuilt in a container

[chriswyatt1]

We can adapt the nf-core dockerfile: https://github.com/nf-core/tools/blob/master/nf_core/gitpod/gitpod.Dockerfile, with the custom tools (e.g. tree and the pulled RNAseq repo).

Then check that all the elements of the training still work, and adapt the training docs to match.