feat(DockerBuilder): implement .dockerignore like behaviour

[nokome]

Currently we send all files in the folder to the Docker daemon as a build context, with a few exceptions:

https://github.com/stencila/dockter/blob/v0.2.6/src/DockerBuilder.ts#L61

It would also be sensible to ignore node_modules etc. See https://codefresh.io/docker-tutorial/not-ignore-dockerignore/

[giorgiosironi]

This is language-specific, but as you say lots of conventions-based folders like node_modules could be excluded.