This was originally posted on the Docker Hub page by acsandeep. Posting it here as well for more visibility.

The run command in the readme file for Docker 1.9 and up includes

-e DB_HOST=bookstack_db:3306 \

However, the variable only needs the name of the host, not the port. The program appends the port when running, so you end up with a

SQLSTATE[HY000] [2002] Failed to parse address "bookstack_db:3306:3306"