Docker config for generic LDAP server

[jonathanmaw]

As of #34, we now have a way to spin up a FreeIPA server and test against it.
This handles most of our needed testing capacity for developing for FreeIPA or LDAP.

However, there are many ways an LDAP server could be configured to store information about users (e.g. how does ActiveDirectory do it?).
When we revisit generic LDAP support, we should have a way of spinning up a generic LDAP server and feeding it some generic config, too.