Add Dockerfile for dev env

By including this Dockerfile and linking this repo to Docker's hub Automated Builds, a user can have an OpenBazaar setup just by running `docker run -d -p 12345:12345 openbazaar/openbazaar` (assuming you choose `openbazaar` as your docker hub username). This is extremely lightweight and a lot faster than Vagrant.

In development mode, devs can use this environment to run their code using the volume mount feature of docker. `docker run -d -p 12345:12345 -v /path/to/dev/repo:/bazaar openbazaar/openbazaar`. Omitting the `-d` flag will cause the logs to be printed in the console.