So, I got it working. It seems it was a problem with the DNS, somehow(because it was able to resolve the url when using the getent and openssl commands). So I ran getent to get the host IP of the SMTP relay and ran the container changing the server address to the raw IP. That got me to a different error, the one in this thread. The solution pointed out there, to add the line DISCOURSE_SMTP_OPENSSL_VERIFY_MODE: none to the yaml file, got it working in the end.
Don’t really know if the DNS thing will cause problems in the future, hope not lol, but thank you both for your help!