I have been using the Joplin server docker image behind an nginx reverse proxy. The last version I got to work was, 2.0.1 beta (using the SQLite db). I also had to use florider's version as the official one always started with data already in the admin account and an additional "user1" account. Even after a complete new, clean ubuntu / docker install!!
I then moved onto the later 2.x series using the same .env file and I always got "Invalid origin" regardless of the version. This suggests that the proxy and the Joplin server are not on speaking terms. If I reverted to the 2.0.1 version of the server it worked again. I am not a fan of the Docker concept and so it is currently a bit of a dark art to me and is likely to remain so. I didn't have the knowledge to troubleshoot the issue so I gave up.
Not exactly helpful but just to let you know that you are not the only one who has come across this... assuming it's the same problem 