Documentation about running tests with Docker

[FrediKats]

I was trying to make some changes in the project. But I faced with issues on running bundle update on my windows machine:

Installing ruby-filemagic 0.7.3 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

I'm not sure that this gem was designed for running on Windows machine.

Can you add instructions for running tests in Docker for Windows users?

[pnatashap]

There is no easy way to install filemagic on Windows, so you can use Docker or WSL (windows subsystem for linux) or Github actions in your fork (tests will be run automatically for PR and you can check the results).

Additional part in documentation is still required