Docker ubuntu version in config

[marchellodev]

Would be really great to be able to specify version of ubuntu that is used in docker.

Sometimes when you build linux application via docker and then run it on older systems, app doesn't work (due to it was built "on a too new system and uses too new libraries")

[provokateurin]

So we should lower the ubuntu version? 16.04 isn't supported since April anymore so I think we shouldn't even support that.

[marchellodev]

That's where this is coming from.
AppImage/appimage.github.io#1968 (comment)

You don't have to lower it, just make it possible to specify version in config file

[provokateurin]

So they say you should use the oldes supported LTS version which is bionic, but using xenial should also work. The thing is we are already using xenial as ubuntu version if I'm not mistaken.