Have a Snapcraft-capable machine
Hint: NixOS doesn't have Snapcraft at the time of publishing.
Be part of the NixOS Foundation organization on Snapcraft:
https://snapcraft.io/nix-base
Then:
snapcraft login./publish.sh
However, good news, you will probably never need to do this since the
base image has almost nothing in it but the minimum necessary
directories plus /nix.