.NET Docker has main and nightly:
https://github.com/dotnet/dotnet-docker
https://github.com/dotnet/dotnet-docker/tree/nightly

The nightly branch updates to new versions of .NET that aren't released yet. This can serve as a staging area for main.

For a Go release branch, main would only contain the "final" 1.16.6 build, for example, but nightly would be updated for every change added to the 1.16 branch up until release day, before 1.16.6 is released.

This lets us validate that the release will work in Docker before release day, and lets our users validate it too.