Allow multiple volumes to be mounted as init scripts for mysql

[mtzmontiel]

Currently this image loads only the first level on path:

 /docker-entrypoint-initdb.d/

So if some wanted to use multiple volumes mounted under that only the lowest level would be executed.

I want to use volumes or to order init scripts in a more meaningful manner:

/docker-entrypoint-initdb.d/00-create-schemas.sql
/docker-entrypoint-initdb.d/10-myschema-seed/10-schema-1.sql
/docker-entrypoint-initdb.d/20-myschema-seed/10-schema-2.sql
/docker-entrypoint-initdb.d/30-myschema-seed/10-schema-3.sql

[tianon]

Closing for the same reasons as the associated PR: #193

[tianon]

(Suggested workaround is to drop a short .sh file instead which sources additional directories.)