If you want to use Docker to build cross-platform images, the first step is to enable QEMU to run images targeted at others architectures via emulation. I assume you have installed Docker CE and its containerd runtime as described here, and are running on a x86_64 host.
The Microsoft .NET SDK is an open-source development platform for developing applications across multiple architectures and operating systems. In this article, I will show you how to install the .NET SDK on Ubuntu 20.04 and then create/compile/run a simple web application. Ubuntu 22 will have the dotnet-sdk available in the default Ubuntu apt repositories, but … Ubuntu: Installing .NET SDK 6 on Ubuntu 20.04