Support Docker-based development environment

Summary:
- Merge Dockerfile.base -> Dockerfile
- "docker build ." now works.
- "fig build && fig up" now works.

The full command I use to build everything from clean sources is:

    sudo git clean -dfx && chmod -R u=rwX,g=rX,o=rX . && git-set-mtimes && fig build && fig up -d && fig logs

git-set-mtimes ensures that we don't build the requirements more often
than we have to.  It comes from here:

    https://raw.githubusercontent.com/docker-library/official-images/0bb1458242ca42b1ff117da9b7aed076c68688fc/bashbrew/git-set-mtimes

Test Plan:
    fig build && fig run inbox tox

(Note: Some tests seem to fail right now for some reason, but this
appears to be unrelated to this change.)

Reviewers: spang, colhom

Differential Revision: https://review.inboxapp.com/D738
config: Support loading config from INBOX_CFG_PATH environment variable

Summary:
This makes it easier to change the configuration from within a
separate development environment.

Test Plan: None

Reviewers: spang, colhom

Differential Revision: https://review.inboxapp.com/D739