Provide Dockerfile for a self-contained poll-server

[frans-fuerst]

A Dockerfile which creates a docker image containing a running poll-server would have several benefits:

• easy setup (especially for those who don't know Ruby/gem/rake/MySql etc.)
• reproducible setup for different environments
• encapsulation of services
• avoids 'pollution' of a working system in case you have to uninstall poll

I tried to setup one based on Ubuntu 15.4 but failed to setup database connection. I don't want to install poll together with Ruby and MySql on my desktop system

[flavienbwk]

After 4 years no news...