I tried installing 3.18.2-x86 in a VirtualBox (7.0.8 on Win host) VM.
The VM was created with all the default settings. The install goes
smoothly: boot from iso, login as root, run setup-alpine, accepted all
defaults but specified a "sys" install to the "sda" drive, which also
appears to go smoothly with filesystem creation. However, upon reboot
and logging back in as root, the /dev/sda3 filesystem is not mounted at
/. Instead / is still mounted as tmpfs. Is there some trick to get
/dev/sda3 correctly mounted after initial install of a "sys" filesystem
setup?