How do I know what my username/password is for SQL PLUS?
I have not given a username or password during the whole process.


Why not? It is clearly written on that site, right before the step where you try logging in with sqlplus.


  
  Execute docker exec &lt;&lt;container id&gt;&gt; /opt/oracle/setPassword.sh &lt;&lt;pwd&gt;&gt; to reset password for SYS user.