Alpine Linux in LXC + docker

Maybe you ...

Sure I would have shared with the OP what resolves his issue if I knew it from countless experience before.

Warnings and errors are - as already said - common with running Docker in LX(C) in PVE.

Not with LXD.

If you don't want to see the error, override the used defaults in /etc/conf.d/docker. This is just a mismatch of the settings in the LX(C) container and the settings from the outside.

And why not fix it the other way around?