Problem deploying dotnet to

I am very new to docker so please forgive me if this is some obvious noob error.

I wanted to move the identityserver instance I have on heroku to render. On heroku I was building the docker container locally and uploading it… Render does not appear to have that option but it detects docker files in the repository and builds them automatically. Unfortunately when I do this I get the error"
You must install or update .NET to run this application
It builds and runs fine locally but the render community said this is an issue with my docker setup.
Full build log community forum thread

Can anyone give me some tips about where to read up on this or what I might have done wrong?

Oh I probably should have linked the docker file

Someone on discord pointed out that my docker file was using
FROM AS base
aspnet 7.0 was released last month. Changing latest to 6.0 fixed it.

A noob error indeed