In order to access McMyAdmin you have to change the default username:
From SSH into the server:
1. docker exec -it mcmyadmin2 /bin/bash
2. ./minecraft/MCMA2_Linux_x86_64 -setpass yourNewUserName -configonly
3. docker stop mcmyadmin2
4. docker start mcmyadmin2
5. login with username yourNewUserName and password password
6. Change the password from the McMyAdmin web interface