Is NEXTAUTH_URL build time or runtime env variable?

[nemanjam]

Documentation about NEXTAUTH_URL variable is very short and unclear.

https://next-auth.js.org/configuration/options

1. It doesn't say anything about is this variable required at build time or run time?

2. What will happen if I leave it undefined at build time?

3. If I set it at build time I don't have to set it at runtime because its embedded in the bundle? Why would I need to define it twice?

4. What if I set one value at build time and another value at runtime?

Very unclear.

Also I noticed that this variable doesn't expand, for example if I set it to NEXTAUTH_URL=$PROTOCOL://$HOSTNAME and log it inside Next.js app with process.env.NEXTAUTH_URL it stays like that although both $PROTOCOL and $HOSTNAME vars are defined. Why?

I use Docker for deployment and not Vercel. I need to be clear about these details because I need to dedicate build argument in Dockerfile if it's required at build time.