Add Support For Connecting to Docker Hosted Backstop Server

[jkusa]

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:

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.

[garris]

This is totally right on. Both of these concepts are something I would love to see implemented.

1. ability to hit a remote backstop (or better yet -- enable backstop to hit a remote chrome -- see https://github.com/garris/BackstopJS/issues/1002 )
2. add a CLI param to override host and port