Readme references wrong base image

[florensie]

In the readme, this is stated:

Choose your base image in Containerfile (line 23):

FROM ghcr.io/ublue-os/bluefin:stable

The base image is actually on line 48. If I understand correctly, using bluefin:stable is not how this template is meant to be used either.

[castrojo]

Mind sending a pr? thanks!