Add Support For Connecting to Docker Hosted Backstop Server

Currently, ember-backstop expects to connect to a backstop server connected on the same host as the test host. It would be really useful to have a docker based renderer server for consistent verification across dev machines and CI.

I was able to prototype this by running the backstop docker image and mounting port `3000` to local `3000`. After that I needed to update the `origin` field in the fetch request to the following:

**Current**:
https://github.com/garris/ember-backstop/blob/c0cd99513a95c9f4d9647d66a925cb5b6e20c7ca/addon-test-support/backstop.js#L139

**Modified**:
```js
origin: `http://host.docker.internal:${new URL(ORIGIN).port}`
```

This is obviously a POC/hack, but it would be great to have first class support for connecting to a docker based server.