Separate web pages source from support code

[jchonig]

Addresses https://github.com/things-nyc/things-nyc.github.io/issues/39

• build stuff (Dockerfile, Gemfile*) live in build
• Web site data lives in docs (standard for github pages)
• Optimize docker image
  • Build on Alpine
  • Combine RUN commands for fewer intermediate images
  • Only all Gemfiles to the image (was including the whole tree)
  • Clean up docker-compose.yml syntax

[jchonig]

Requires changing the publishing source on the github pages from / to /docs. See step 7 at Configuring a publishing source...