OK, so it definitely seems something to do with the docker and the proxy or dns settings then. Can you ping/wget other addresses inside the container, though I assume that you won’t be able to.
I have just built here with the same docker configuration as on github and it manages to contact the repo.
I’m not sure really what to suggest as docker and proxies can be a pain to get working and it really is porbably going to just be a case of trying things that come up from google until it works