Activate Unity3d and run tests in docker image with gitlab-ci

Now that we have tests with #2 (closed), let's add a .gitlab-ci.yml and grab test results ran inside.

• Create the .gitlab-ci.yml
• Confirm Unity3d loads and runs correctly
• Have a working test command (tested locally)
• Login successfully using environment variables
• Activate Unity3d (see related Unity3d Q/A)
• Have the tests executed
• ⚠ Load nunit test results (*results.xml) edit: will do in an other issue