Changed IsDocker check (#1998)

* Refactored IsDocker to be completely static and changed to use an environment variable instead.

* Removed file from another branch

Dockerfile

@@ -32,5 +32,7 @@ WORKDIR /kavita
 
 HEALTHCHECK --interval=30s --timeout=15s --start-period=30s --retries=3 CMD curl --fail http://localhost:5000/api/health || exit 1
 
+ENV DOTNET_RUNNING_IN_CONTAINER=true
+
 ENTRYPOINT [ "/bin/bash" ]
 CMD ["/entrypoint.sh"]