The easiest is just read css file with c# and its content in <style>...</style>
Another option is to reference it with file:/// in HTML head. Just make sure jsreport is configured to reach your file system
Another option is to call this
with proper HTML you can reference css/js that are in your project. https://jsreport.net/learn/base
This project (jsreport) has nothing to do with jasper reports.
The link you provided (jsreports) is from a completely different project, which is actually not being developed for some time as far as I know.
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