Pass `$SINGULARITY_` env variables through

This is a continuation of #1386

Basically I want to set up some $SINGULARITY_* configuration variables in the environment to not require modifying config files.

Specifically I want to work with encrypted containers in one specific invocation and am hence setting $SINGULARITY_ENCRYPTION_PEM_PATH

Now NextFlow filters this variable removing it from the environment when invoking singularity. I do like the proposal to allow any SINGULARITY_* variables by default.

One solution would be to write a configfile passed via -c:

singularity {
  envWhitelist = 'SINGULARITY_ENCRYPTION_PEM_PATH'
}

But I'd like to avoid that as I'll need to pass an additional param to each invocation of NextFlow.

Given that there is singularity.cacheDir being the same as $NXF_SINGULARITY_CACHE_DIR I expected $NXF_SINGULARITY_ENVWHITELIST to work but the handling of the cache dir is a special case and only works for that.

Is there any chance to simplify passing Singularity options via the environment/globally?

Start with Nextflow's Singularity configuration handling, especially envWhitelist and the special singularity.cacheDir/NXF_SINGULARITY_CACHE_DIR mapping described here. Determine whether SINGULARITY_* variables should pass by default; done means the requested variable reaches a Singularity invocation without a per-run config file.