I will just add on that I made sure targetframework was .net9.0 in all projects. It didn't build, just complained about The current .NET SDK does not support targeting .NET 9.0. Either target .NET 6.0 or lower, or use a version of the .NET SDK that supports .NET 9.0.

Just changing base and build in the DockerFile from 6.0 to 9.0 fixed the problem. The error message wasn't even close to what the issue was.