For the record, I hit that issue too. /boot got full. However, I could not fix it with the solution at https://forum.cloudron.io/topic/1310/disk-filled-up-with-kernel-update-artifacts/2 . I had to follow https://askubuntu.com/a/430944 (slightly adapted)

I'd love to see this feature too! Sadly, there is no way to do such a thing yet.

Two possible workarounds for your problem:

• Create a basic admin interface under /admin or something, that would allow you to configure your proxy
• Assume that someone who configures a proxy is at ease with a command line, and just have a config file under /app/data that the user can configure with the webterminal

Sorry, it currently is not.