doc: add clarification for length limit of init password (#10824)

docker/.env.example

 
 # Password for admin user initialization.
 # If left unset, admin user will not be prompted for a password
-# when creating the initial admin account.
+# when creating the initial admin account. 
+# The length of the password cannot exceed 30 charactors.
 INIT_PASSWORD=
 
 # Deployment environment.