Add docker support in pipeline download

[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

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

[MatthiasZepper]

Looks like this will run into merge conflicts as there are multiple refactors running concurrently (
#3509, #3506)

😱 Yes, that is likely inevitable! Unfortunately, I did not know that you were working on this task at all! We coordinated the other refactors via Zoom and Slack and I have asked multiple times in the channel that people working on tasks should come forward. I was online on Monday and Tuesday until 1.a.m. UTC so people from the Americas could reach out if help was needed.

😰 This is now very unfortunate, but it seemed to be the easiest for testing and maintenance to move all Singularity download functions into a separate class and build an analogous Docker download class later. Do you think that you could move your work into a similar class like in #3509? Basically, one that just takes the container strings and registries and then downloads the container images?

[nikhil]

No worries! I must have missed that on the hackathon slack channel, I added a draft PR of my work into a docker class using the singularity class as a base. Its located here: #3517. Should be good to add once the other PR is in

[nikhil]

I am going to close this PR since this should not get merged