Docker-wrapper

Fix
- Remove color=never
- Fix failing test

Fix
- Set login and tty options to false as default

Feature
- Add tty option
- Add login option in run

Fix
- Remove whitespace removal from run()
- Use docker exec's return code

Breaking
- Removal of whitespace prefix and suffix from stdout and stderr is removed.
- As this relies on https://github.com/docker/docker/commit/00c2a8f323548b7d0aa54cfd10a594dd93ddbed0 it won't work with older Docker versions than 1.3.2.

Feature
- Use cat instead of echo for write_file

- Fix environment variables in commands
- Add more information to DockerUnavailableError