Data volumes for configuration, secrets; new ./go commands

[mbland]

This is still a work-in-progress, but there's enough changes that merit a review at this point. Happy to have this merged, then follow up with another PR to complete the steps for other images (notably for the Team API and the Hub). I'm focusing on getting the core Pages functionality up now.

I need to document this better, but the convention is that for each image:

• There's a config directory that gets mounted as a data volume by ./go run_daemons or ./go run_container.
• ./go run_container will run the Docker command to give you a shell inside the container; ./go run_daemons can run one or more of the containers as a daemon, which you can inspect with docker inspect or docker logs or docker ps.
• Any secret configuration files in each IMAGE/config directory are listed in .gitignore so they aren't accidentally checked in.
• The most convenient secret file format is IMAGE/config/env-secret.sh; the images with this run /bin/bash -l -c 'source env-secret.sh && COMMAND'. (I may eventually update Pages to depend on a secret env var, and want to add OAUTH2_PROXY_SIGNATURE_KEY to bitly/oauth2_proxy.)

After this, I'll send a PR for sharing a data container between the pages and nginx-18f containers, so that the pages container can generate sites and nginx-18f can serve them.

cc: @jcscottiii @ccostino @afeld

[ccostino]

Awesome @mbland! Just had a couple of questions before sending it along. :-)