Replace custom Docker code with library calls

[andystanton]

Ticket #35 details changing the shelled out calls to the Docker CLI to http calls to the Docker remote API. This is admirable and I got this working, but ultimately there is no point maintaining this code which someone else has already written better. In this case, the Docker developer documentation mentions two mature libraries that perform this function.

This ticket will involve researching which is a better fit for dexec and then switch dexec to the chosen library.

[andystanton]

I've decided on https://github.com/fsouza/go-dockerclient