Why add ?ephemeral=true to TS_AUTHKEY?

[huyz]

docker-guide-code-examples/02-oauth/compose.yaml

Line 9 in fe530b2

- TS_AUTHKEY=tskey-client-kwLoXj6CNTRL-vCLN9Ab8QYYoLSEM98riXYLnfmtej6Lh?ephemeral=true

has ?ephemeral=true appended (also as instructed on https://tailscale.com/kb/1282/docker#ts_authkey)

But why? When we create an Auth Key, we already designate it for "Ephemeral" use. Indeed, the UI shows that the container is noted as Ephemeral. So why the extra ?ephemeral=true?