Add test version of tvb-widgets to Dockerfile.

Hello, we would like to test the tvb-widgets package on EBRAINS lab-int before doing an official release. We released it only on testpypi for now, thus I have added the corresponding installation command in the Dockerfile. After the official release, we will remove this change from the Dockerfile, and just add the library to the requirements.txt file.

Would you agree to this temporary change? If yes, when should we expect to have the package available on lab-int?