If you're using Java Spring Boot, you can set the server port the application runs on to any port, but also to a random port:
server:
port: 0
However, when I now use a Dockerfile for my application, I won't know what port to EXPOSE. Is there any way to find this port?
when you started an app, you can see at console I think. As I know docker use 8080 or 8081.
EDIT 1: Check this -> link
You can expose a port through your Dockerfile or use --expose and then publish it with the -P flag. This will bind the exposed port to your Docker host on a random port (verified by running docker container ls).
You can expose a port through your Dockerfile or use --expose and then publish it with the -p 80:80 flag. This will bind the exposed port to your Docker host on port 80, and it expects the exposed port is 80 too (adjust as necessary with HOST:CONTAINER).
You can ignore exposing anything and just use -p 80:80 in which case this doubles as both exposing AND publishing the port.