After two days of experimenting I managed to get the FTP function of my Sony alpha a7IV to work for the automatic upload of pictures.
My current setup is a SFTPGo as FTP server with active TLS, to which the camera can connect. The camera itself is connected to the hotspot of the cell phone for internet connection.
I have noticed a few limitations so far.
A not so clear one is that only RSA certificate can be used, because according to my current knowledge no ecdsa certificates work (my root CA is of course only ecdsa).
Furthermore there is the limitation from the camera side that apparently no IPv6 is supported.
A first simple docker-compose file can be found in my GitLab.