*Following from: https://blog.henrici.name/blog/2021-01-26/
Alpine Linux does not behave like other Linux distributions. Single user mode is not immediately helpful
Linux can be booted in single user mode by appending “single” to the kernel command line. It can be added by modifying that command line on the bootloader prompt.
Alpine Linux, however, does not mount the root filesystem when doing this. And in the /dev folder the root file system device is not yet populated.
First populate /dev properly:
nlplug-findfs -p /sbin/mdev
Then the root filesystem can be mounted (change /dev/vda based on your machine):
mount -t ext4 /dev/vda /sysroot
vgscan vgchange -ay