Dockerfile

Sorry for the commit mess. Thankfully github is good at summarizing. That's just me not knowing how PRs and forks work with github yet.

This dockerfile uses the ["trusted" Node.js Build](https://registry.hub.docker.com/u/dockerfile/nodejs/) and the latest release tarball. I suppose it could clone from git if that is prefered, but I think it makes more sense to use the released version as that is less likely to break on some random day.

```
$ sudo docker build -t benweet/stackedit
$ sudo docker run -d -p 3000:3000 --name stackedit benweet/stackedit
```