Dockerfile to build collins docker image

[discordianfish]

This provides a Dockerfile and some docs to run collins on docker.

IMO this makes testing/trying out and deploying collins much easier.

(@oshu: You might be interested in this :))

[william-richard]

Hi,
Do you think you could add a bit more explanation to your readme/markdown document? Perhaps a bit explaining what Docker is, and that it is a tool that you can optionally use to build and interact with Collins? I want to avoid new users getting confused, especially since it isn't strictly necessary to use Docker, the documentation doesn't mention it.
Thanks!

[discordianfish]

Like this? Let me now if there is still something unclear.

[william-richard]

Much better - I'm going to make one very minor edit and merge it in.
Thanks for the contribution!