Executable Docker container for `mitmproxy` to make running the tests easier

[schlessera]

Is your feature request related to a problem?

Right now, if you want to run the tests for the project, you need to install Python 3 on your system as a prerequisite to then install mitmproxy via its pip3 package manager.

Describe the solution you'd like

Adding an executable Docker container that bundles Python3 would remove the requirement for Python 3 on your host system (trading it for Docker, which is more common), and will allow a more automated approach to onboarding for running the tests.

• [x] I intend to create a pull request to implement this feature myself.