Add/docker

[pbnj]

Checklist

• [x] This Pull will not add the same thing as another currently-open request.

• [x] Your Pull was made against the rivermont:dev branch and not rivermont:master.

• [x] This Pull does not commit any keys, passwords, personal data, or other private information.

• [ ] I updated lines 20 and 21 in the README to reflect any changed I made.

• [x] I wrote at least some documentation for this feature.

• Added a Docker tutorial in README.md

• Added Dockerfile that builds the image.

• Added a docker.cfg (variant of default.cfg) to write files under /data/ path in the container.

Demo of building the Docker image and running it: Spidy Docker Demo

Closes #32 .

[pbnj]

I might need some clarification on what needs to be updated/added on lines 20 and 21 in README.

This is what I see in my editor for lines 20 and 21:

[rivermont]

Guess the README changed again. Looks like it should be lines 15-16. Looks amazing other than that!