Write permission error when running "download" with docker as non-root user

Description of the bug

I get a permission error when running the nf-core download command through Docker with the -u option to set the user & group IDs (as described in the docs https://nf-co.re/tools/#installation).

This is a minor problem, because it works fine without -u.

Steps to reproduce

Steps to reproduce the behaviour:

1. Command line:

docker run -itv `pwd`:`pwd` -w `pwd` -u $(id -u):$(id -g)  nfcore/tools:dev download --singularity-cache-copy -c singularity -x tar.gz rnaseq


? Select release / branch: 3.4  [release]
? Define $NXF_SINGULARITY_CACHEDIR for a shared Singularity image download folder? [y/n]: n

2. See error:

AssertionError: Command 'nextflow config -flat nf-core-rnaseq-3.4/workflow' returned non-zero error code '1':
[red]> mkdir: cannot create directory ‘//.nextflow’: Permission denied
mkdir: cannot create directory ‘//.nextflow’: Permission denied

Expected behaviour

It downloads the pipeline.

System

• Hardware: Mac Mini
• Executor: Docker
• OS: macOS 11.6.1
• Version of nf-core/tools: nf-core/tools version 2.2.dev0 (and earlier)
• Python version: N/A

Start by reproducing the documented docker run ... nfcore/tools:dev download command with -u, then trace the download entry point where it invokes nextflow config -flat. Check how the working directory and .nextflow location are selected for a non-root user; done means the command downloads the pipeline without the permission error.