Contrast system command wrappers

[psychtek]

Given most functions are wrappers to terminal commands and to support Darwin, Linux and Windows system need to contrast which system funs best to use for Docker:

• processx::run()
• system()
• sys::exec_wait()

[psychtek]

Went with the processx class. Allow for future system background checks.

• Add healthcheck to containr_start
• Capture stderr etc to improve starting messages from background process

[psychtek]

#9 makes use of the processx package.