Docker Runners won't work behind a proxy

Hi,

I have an issue with the runner.

I have registered it sucessfully and it accepts new jobs, but after cloning the repo it always gets stuck on checking out the master branch.

Running with gitlab-ci-multi-runner 1.4.1 (fae8f18)
Using Shell executor...
Running on 615APC615000366...
Cloning repository...
Klone nach '/home/hannes/builds/d5371889/0/rosenoeggerh/test-ci2'...
Checking out 5b3d4425 as master...
ERROR: Build failed: execution took longer than 120 seconds

Doing this should not take longer than a few milliseconds, because that repository only contains an empty readme file and a .gitlab-ci.yml with 3 lines.
I'm sure this is an issue with git somehow.
Do you have any idea how I can debug this further?

Checking out that commit when I clone the repo manually works like a charm.