Support for bulker multainer

[kopardev]

New feature

Multainer support will really enhance the utility of Nextflow by potentially decreasing the number of processes in any given workflow. May even improve efficiency as more work will get done on any given compute instant and wait time between computes will be low.

Use case

Often times we need multiple tools within a given process. So we have to split the process into different cascading processes where each process uses a different tool-specific docker. To avoid this we could support multainer like bulker

Suggested implementation

[bentsherman]

Related: #2527