How to create an AWS CodeBuild Custom Build tool from a .Net Console App

Question

I wrote an .Net console app (.Net Core 5) that converts rtf documents to html pages. I build and run it locally in a docker image. I want to make my docker image into a custom build image for AWS CodeBuild. I created the CodeBuild project, it provisions the image and all that. But I need to run the command dotnet myproject.dll after CodeBuild presumably from the buildspec file. I have fought a few different errors. What I am looking for a resource that would show me how to do this.