Wordpress: Error establishing a database connection

[NeuroNex]

1.16 Running a WordPress Blog Using Two Linked Containers
7.1 Using Docker Compose to Create a WordPress Site

Both examples ends up with "Error establishing a database connection" when opening http://localhost
Test environment:

• Ubuntu 14.0.3, 64bits
• Docker 1.91
• Docker Compose 1.51

[sankerkr]

mysqladmin: connect to server at 'localhost' failed
error: 'Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)'
Check that mysqld is running and that the socket: '/var/run/mysqld/mysqld.sock'
exists!

Getting the following error