fix(Dockerfile): Install requirements separately

I don't fully understand why, but when installing teeth-agent directly and having a git+http url in the requirements.txt, it's unable to find the teeth_rest requirement via git url. 

Installing the requirements separately works. 

Some articles on the internet implied this was because the requirements.txt is parsed by setuptools, which doesn't understand that URL type. I'm not 100% sure that's correct, but I do know installing requirements in requirements.txt in a separate step fixes the issue.