It seems you have twice FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base in your dockerfile.
It should be there just once. Maybe the apt-get part isn't running actually. Please remove the second FROM declaration and try it again.
You need to make sure the extension is properly loaded into the solution. You do this by installing the extension with npm inside of the jsreport folder. And also make sure that your jsreport.config.json file has the key/value for "discover": true