adding support for custom network args

[vsoch]

This will close #60 @scicco you can follow the example here: https://github.com/singularityhub/singularity-compose-examples/tree/master/v2.0/custom-network

I think part of the trouble was the entrypoint had an envar, evaluated from the host is not going to work, so I added a startscript instead. So this PR is adding support for custom network args primarily.

Give that a shot, show me what doesn't work (and I'll fix it up) and we can go from there!

Signed-off-by: vsoch [email protected]

[scicco]

First of all thank you for the PR 😄

I've checked the example and I have some doubts. I'll try to illustrate them by commenting on your changes.