Reading the Next.js documentation about environment variables -

In order to keep server-only secrets safe, Next.js replaces process.env.* with the correct values at build time.

Does it mean that the environment variables available at build time are the only variables are available for Next.js application at runtime? This seems to be problematic as it means I will have to build for each environment!

And, if that is the case, how do I enable my application to adopt to 12-factor delivery? I want to package Next.js application as a Docker image which I must build only once and then deploy to three different environments - test, staging and then production on Azure PaaS.