Update Dockerfile to modify correct config file

[ltangvald]

MySQL 5.7.14 changed the structure of the configuration directory, so
/etc/mysql/my.cnf no longer contains the config options, but only includes other directories.
This caused the modification done to the config in Dockerfile to fail silently, creating issues
with any setups that required those changes, such as running with a different user.
Changed script to modify the new file /etc/mysql/mysql.conf.d/mysqld.cnf

Fixes issue #197

[yosifkit]

It looks like the default mysql install now listens publicly. We need it in the docker image, but won't that be unexpected for other users of the mysql apt repo?

Do you think we should also move the skip-host-cache\nskip-name-resolve to a separate config file so it is easier to remove? Hopefully to help with #154.

[ltangvald]

Yeah, that's a good point. It's more in line with the default config on other platforms, but it's potentially a breaking change and shouldn't be changed for a GA. Thanks for pointing it out :)
Moving the docker-specific options to a new file sounds like a good idea.

[tianon]

Arg, bind-address is now back, and breaking 5.7. 🍭

Merging as-is to fix that! (and blaming @yosifkit appropriately for pointing it out and probably prompting you fine upstream folks to fix the issue 😄)