Add /docker-entrypoint-initdb.d/ functionality

[IvanHorpynych]

Hi!

Sorry to distract you, but could you, please, add functionality for the basic initialization of DB?

Like in this project: https://github.com/nkonev/docker-h2/blob/master/docker-entrypoint.sh

[flyasky]

Up Vote)
or like here https://github.com/buildo/docker-h2database