So what Puffrfish also said before - you can not add a plugin.
But when youy create a server with a exist server type and then go with SFTP to the server files and replace the server.jar with a spigot variant it works.
Solution: After pinpointing the problem (404 error) I figured that my Nginx "pufferpanel" config was invalid. The error was due to the fact that in the "location /assets" block I had the following: "try_files /app/&uri =404;" instead of "try_files /app/$uri =404;". After changing the & to $ and then reloading Nginx everything worked as intended.
That does appear to be correct except for lacking the default votifier port of 8192. I can only assume from that config that you've changed the port that votifier is running on.
After re-creating the docker container and starting it, run the following and ensure that the ports actually got mapped correctly.
# docker ps -a | grep bungee
If they are not mapped correctly, you'll have to destroy and re-create the docker container manually. You can find out how to do that at the following link. https://community.pufferpanel.com/topic/31/an-unofficial-faq-tutorial