Proxy execution in non-native mode via script

[alfpark]

Proxy the execution of the non-native mode Docker/Singularity command via a script executed on the host. This will reduce issues with command line quoting, env vars, post-task execution steps instead of an in-line command. Further improves upon #234 and a pre-requisite for #230.

• Migrate key portions to pre-exec env vars
• Create proxy execution script
  • Dump env vars for Docker/singularity
  • Pre-execution step
  • Task-execution step
  • Post-execution step
• Task termination tests