Solution:
I have solved it by deleting everything and try starting it up again.
docker rm -v spawning-pool # -v Remove the volumes associated with the container
Remove the volume too
docker volume rm hatchery
Then I created the containers again
# create the volume
docker volume create hatchery
# MySQL instance
docker run -it -d --restart on-failure -v hatchery:/var/lib/mysql \
-e MYSQL_ROOT_PASSWORD=Kerrigan \
-e MYSQL_DATABASE=zerglings --name spawning-pool mysql
# creating wordpress
docker run -d --name lair -p 8080:80 --link spawning-pool:mysql \
-e WORDPRESS_DB_HOST=mysql -e WORDPRESS_DB_NAME=zerglings
-e WORDPRESS_DB_PASSWORD=Kerrigan wordpress