RFE: Add environment variables option

[asmacdo]

IMO, containers should usually be configured via environment variables, so it seems like
datalad containers-run should accept --env and --env-file and pass along to singularity.

Alternatively, we could also add a flag --singularity-vars "env-file=/home/here" or similar.

[yarikoptic]

yeah, might be useful to allow for custom env configuration for a container. In principle that can already be possible for people by tuning their fork of this dataset and changing invocations in .datalad/config accordingly. Before bothering to improve upon that we better gain specific use cases to address.

[asmacdo]

My only use case was experimenting. closing for now, if anyone needs this please reopen and post your use case!