Configuring docker remote interpreter gives error "Can't create Python SDK"

I am running PyCharm PE 2017.3 and try to connect to a remote Python interpreter based on Docker.

In the settings dialog PyCharm conforms that I can connect to the remote machine via port 2375 over tcp.

I have selected a python:3.5 image from Red Hat which is available on the Docker VM and can be run remotely via shell command. So this works:

sudo docker run -h tcp:<hostname>:2375 -it --rm <registry-url>/python:3.5 bash

However if in Pycharm I click "OK" in the settings-dialog Pycharm freezes for a while and then an  errorbox appears with title "Can't create Python SDK" and message "Network timed out while trying to connect to https://index.docker.io/v1/repositories/library/busybox/images. You may want to check your internet connection or if you are behind a proxy."

Indeed I am working in an Enterprise environment which has no internet connection, but that should not be necessary because the Python docker image I am using is available in our private Docker registry.

So what do I have to do to get his working?

Any help appreciated