Add NXF_DOCKER_CACHEDIR to support offline docker images

[nikhil]

New feature

Essentially this is the docker equivalent of NXF_SINGULARITY_CACHEDIR. If this env is set it will load the docker images saved in this folder.

This feature is the result of a nf-core tools refactor from the hackathon: nf-core/tools#3517

The guide for loading and saving docker images is here: #4708

Use case

This will allow users to create a pipeline in an offline docker workspace for users who don't have access to singularity.

Suggested implementation

Nextflow will just need to run the load command on the tar files in the folder. Further details are here: #4708

[pditommaso]

It looks a too niche and, above all, error prone process. We are not planning to support this capability.