Add wait for the database to be up

As there is now a PostgreSQL container that the api container talks to,
have to wait for the database container and application to be available
before starting the api.

The `wait-for-it.sh` script is the accepted way to wait for other
services to be available before starting.

Author: ssoriche

Dockerfile

@@ -24,4 +24,4 @@ USER metacpan-api:users
 
 EXPOSE 5000
 
-CMD [ "carton", "exec", "morbo", "-l", "http://*:5000", "-w", "root", "./bin/api.pl"]
+CMD [ "./wait-for-it.sh", "db:5432", "carton", "exec", "morbo", "-l", "http://*:5000", "-w", "root", "./bin/api.pl"]