Experiment with the Libcontainer trait for `build_container` to something like `open_stdio`

[jsturtevant]

          `build_container` seems like a fairly heavy trait function.

I think if we add something like open_stdio to the trait we can implement common build_container?

Originally posted by @cpuguy83 in #226 (comment)

[jsturtevant]

This might help with #240 as well

[Mossaka]

The question is - do we want shim authors to be able to build their customized Container using Container's builder methods?

[jsturtevant]

There is an approach in #250 that simplifies this