Add docker downloads

[nikhil]

resolves: #2309

This PR adds docker support to the pipeline download functionality

• Many of the singularity functions were made container agnostic and reused for docker purposes
• Looks like this will run into merge conflicts as there are multiple refactors running concurrently (Moved all singularity-related download functions into a new file #3509, Move WorkflowRepo class into seperate file, since it will be shared b… #3506)

This PR uses the guide to download the image: nextflow-io/nextflow#4708

It mimics Singularity's cache env variable by using the new env variable NXF_DOCKER_CACHEDIR, the downside is that this is not an official env in nextflow and the user needs to run

ls -1 [$NXF_DOCKER_CACHEDIR]/*.tar | xargs --no-run-if-empty -L 1 docker load -i

on their own as nextflow currently has no way of loading offline tar files

This PR is a rework of #3516, to use the new container class created here: #3509

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Attention: Patch coverage is 30.28571% with 366 lines in your changes missing coverage. Please review.

Project coverage is 75.77%. Comparing base (33830f9) to head (72b6d60).
Report is 20 commits behind head on dev.

Files with missing lines	Patch %	Lines
nf_core/pipelines/downloads/docker.py	0.00%	200 Missing ⚠️
nf_core/pipelines/downloads/singularity.py	39.17%	163 Missing ⚠️
nf_core/pipelines/download.py	90.90%	2 Missing ⚠️
nf_core/pipelines/downloads/utils.py	97.14%	1 Missing ⚠️

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[MatthiasZepper]

Thank you, Nikhil for refactoring #3516 into a separate class for Docker downloads and directly basing it on the other PR. Much appreciated, and apologies again for the fuss and my coordination failure.

I will focus getting the refactored Singularity download in first, but then get back to this PR to review and finalize the Docker support. Once that becomes available, it will be the first major new feature for nf-core pipelines download in several years, so thank you very much for working on it!

I think, it would be very useful to have Nextflow supporting the NXF_DOCKER_CACHEDIR environment variable that you are proposing. You should open an issue on the Nextflow repo for that!

[nikhil]

Sounds good, thanks so much for working on finishing this up!

[nikhil]

Hey, I added a feature request for NXF_DOCKER_CACHEDIR here: nextflow-io/nextflow#5938