> apk add qemu-guest-agent

ERROR: unable to select packages:
  qemu-guest-agent (no such package):
    required by: world[qemu-guest-agent]

Only way: Change Alpine repositores.

# https://wiki.alpinelinux.org/wiki/Repositories

> setup-apkrepos -cf

or

http://alpine.ccns.ncku.edu.tw/alpine/v3.20/main
#http://dl-cdn.alpinelinux.org/alpine/v3.20/community
http://alpine.cs.nycu.edu.tw/v3.20/main
http://alpine.cs.nycu.edu.tw/v3.20/community