Add docker and singularity images, and environments (conda etc)

[gaow]

In addition to what @stephens999 and I recently listed for future improvement of DSC, I want to add that we can put in docker and singularity support. This should take a couple of days work, with possibly half of time spent on testing (interface robustness and particularly permissions related issues). I'm not very enthusiastic myself about using conda environments (semi-reproducible and difficult to keep track of for me) but if it helps other people we can add that too.

The trickiness is we'll have to rely on individual container or conda environment to save module output formats. This can be problematic, particularly with current RDS / rpy2 vulnerability.

Update: also see #117