Set up docker buildpack on heroku

[colinxfleming]

https://devcenter.heroku.com/categories/deploying-with-docker

can proof of concept this for now but should add it to our heroku pipe at somepoint.

note that we absolutely should not make this user-facing, or put real data into it, just because it's up - needs to undergo a quick and dirty security review first.

[colinxfleming]

depends on #10

[amackcrane]

@colinxfleming I tested this with a simple heroku.yml starting shiny via Rscript (which worked), before I realized rocker/shiny uses shiny-server.sh by default and that might be the preferred invocation. But shiny-server.sh seems like it will run into trouble with the way heroku wants to set $PORT and user. (on my first try it gets permissions errors). Want me to make a PR with the Rscript version?