Fresh install - admin cannot sign on

After three years of nextcloud, I’m still only using the filesync. Maybe it’s time to move away from bloat …

So I spun up the docker-compose.yml from the server manual. The only things I changed were the passwords and the email address (as per the documentation).

When I typed the ip into the browser, it got redirected to path /accounts/login/?next=/ and the sign-on page appeared. Horray!

Unfortunately, the admin email address and password from docker-compose, was rejected from signing on (Incorrect email or password).
So I tried the Forgot password? link and got:
Failed to send email, email service is not properly configured, please contact administrator.

Could someone help me with the fault-finding? Again, I deployed the yml file as-is.
This is on a Debian server.

Here is a generic answer, that can help:
You may need to enter the docker container and execute the script in the respective seafile folder.