Untangling Entrypoint/Cmd parameters for docker

Ading explicit parameters regarding command joins and entrypoint
overriding. Overriding is necessary because there is no easy way to
clear already set entrypoint.
Docker container cleanup

Removed unused Container properties and moved image inspecting to
separate method.
Test make_cmd
test_container
Merge pull request #52 from rabix/refactor/entrypoint

Refactor/entrypoint