Author here. To add or manage VPN users, you can modify "run.sh" and build a new Docker image from the source repo on GitHub. Please refer this README [1] for more info.
Any issues you're aware of with making an arm version of this container? Would love to drop this on my RPi to get an easy VPN pipe back into my home network.
I haven't looked into this yet, but I think it could work on the latest Raspbian 8 [1] which is based on Debian Jessie. You are welcome to clone the source repo on GitHub and give it a try.
[1] https://github.com/hwdsl2/setup-ipsec-vpn#manage-vpn-users