Fix Dockerfile

For security reasons, Dockerfiles have no access to parent directories, so we can't do ADD ../../ or something like that.
If having the Dockerfile in root is a no go for you, I can change it to do a git checkout in a RUN statement. Although it's better to use ADD because:

- you can build collins without depending on github as long as you have the code
- you don't need to check out the repo twice
- ADD is a cache breaker, RUN not. So there is no easy way to rebuild collins without rerunning all steps in the Dockerfile