I’m running into this same issue with the same goal, host Ubuntu 18.04. I have been unable to get Wireguard working in a container with anything I’ve written myself or grabbed from posts. I’ve tried the LinuxServer wireguard image as a client but am getting the same erros as @sebdanielsson . I can’t say for sure that’s the issue, but is the only thing I can see that doesn’t look right.
I’m willing to help debug this though, and will continue to look into it myself!