Add support for "/docker-entrypoint-initdb.d" similar to PostgreSQL's implementation

[tianon]

This also cleans up some minor unnecessary bits in docker-entrypoint.sh so that it's more consistent.

Compare to https://github.com/docker-library/postgres/blob/a82c28e1c407ef5ddfc2a6014dac87bcc4955a26/docker-entrypoint.sh#L49-L97

Fixes #24
Closes #87 (implements what was additionally discussed there)

[tianon]

cc @ltangvald @yosifkit

[tianon]

By using $! instead of parsing the mysqld config, we can guarantee that the process is a child of our shell and thus we can wait on it during shutdown (see below) instead of looping while we wait for it to die.

[yosifkit]

LGTM

[funkyfuture]

Are there docs on this functionality?

[yosifkit]

Looks like we forgot to do that. it should be fairly similar to the one for postgres: https://hub.docker.com/_/postgres/#how-to-extend-this-image

[ltangvald]

Very nice! The upcoming http://dev.mysql.com/doc/refman/5.7/en/shutdown.html might help clean up the entrypoint script further.

[thaJeztah]

Thanks @ltangvald! And thanks @tianon I think this could make many people happy

[funkyfuture]

here's one: 💃

[tianon]

@ltangvald oh interesting -- I'll bet we could just use mysqladmin directly right now, couldn't we?

[ltangvald]

No, unfortunately I got a bit too excited about it :)
mysqladmin shutdown will send a shutdown signal, but won't wait for a complete server shutdown.
So the SHUTDOWN statement won't be very useful as it is now. We'd need to put everything into a single init-file with the syntax constraints that imply.