Refine phabricator deployment

Summary:
Resolves miscellaneous issues discovered in deployment.

- enable LDAP (Resolves T2)
- always upgrade schema before starting
- provide a mechanism for setting the base URI
- add READMEs for Docker Hub

Test Plan: I've already deployed these changes.

Reviewers: #developers, dan

Reviewed By: #developers, dan

Subscribers: dan

Maniphest Tasks: T2

Differential Revision: https://phabricator.paperg.com/D3