Heptapod CI: using our own registry to avoid Docker Hub rate limits
This is a quickfix, we'll probably rehost the image in `heptapod/ci-images` at some point.
There will be about 1h of downtime on 2020-03-02 at 13:00 UTC+1 (change of servers and upgrade to Heptapod 0.20.0 final)
This is a quickfix, we'll probably rehost the image in `heptapod/ci-images` at some point.