Make docker image for nf-core/tools

[ewels]

We have a root level Dockerfile in the tools repo which automatically builds at Docker Hub but it's the nfcore/base one that is used as a base for pipeline images: https://hub.docker.com/repository/docker/nfcore/base

It would be good to also write a Dockerfile that builds the nf-core/tools package into an image that people can use (yes I know, it should be on BioContainers too, but doing docker run nfcore/tools is nicer).

• Build a Dockerfile for tools which has as small a footprint as possible
• Organise Dockerfiles into directories so we can have both base and tools
• Remove DockerHub automated builds
• Write GitHub Actions builds + push to DockerHub
• Write some documentation

[phue]

Could be worth considering micromamba as base image for the container builds. Its size is currently just 31MB (vs 150MB for the miniconda base image)

[ewels]

Yup, I was also wondering about just not using conda. I build the MultiQC image using pip and it's pretty small I think. Maybe also using multistage builds?

Also need to install Nextflow into this image and check around for any other dependencies.. 🤔

[ewels]

Done in #996 - needs checking that the image works! 'cc @ErikDanielsson

[ewels]

Docs: https://github.com/nf-core/tools/blob/bf83b05745d094ac922499f15b29374c9ff2f16c/README.md#docker-image