Use Alpine 3.21.0 image

[user451421541757324]

Alpine has recently released version 3.21.0.

I think the transition to the higher version is not too problematic (most of the steps in the guide work).

This one for me doesn't work with 3.21.0:

sed -i -E 's/(local kernel_opts)=.*/\1="console=ttyS0"/' /sbin/setup-disk