Added Dockerfile

Hello!

You have done a good job with PyTox! I plan to use it soon for one of my projects :)

This pull request contains a Makefile and a Dockerfile that will make it really easy for new contributors to get PyTox running within seconds.

For developers, this will make it easy to have a pristine environment everytime you run your tests.

- ```make run```: starts an interactive container with PyTox already installed.
- ```make test```: starts PyTox's tests in a container
- ```make echobot```: starts the echobot in a container.