is it possible to hand over the credentials in the HTTP Request that opens the Node-Red Website ?
Something like: http://user:firstname.lastname@example.org/ ?
Because I am using 15 different docker containers with different node-red instances and I have to enter the credentials manually each time I have to login ... and that sucks
Only if you are using basic authentication. And if you do that with http rather than https, it would be better if you didn't bother since that is wide open to credential sniffing attacks.
Do yourself a favour and install a web or proxy server to act as the authentication front-end and force node-red to only be accessible via that. Lots of ideas in the forum for that approach.