Support running maze runner locally

[imjoehaines]

Goal

This PR contains various changes to support running Maze Runner locally instead of using the docker image

Changeset

• the fixtures now publish their port so they are reachable outside of a docker network
• various thing such as the hostname or port of the fixtures now supports both dockerised and local versions (e.g. on docker the host is rails6 but when running locally it'll be localhost)
• when running outside of docker the gem needs to be built, which happens automatically in the env.rb file

Testing

Maze Runner is installed in a separate Gemfile to our other dependencies and can be installed by running:

$ BUNDLE_GEMFILE=Gemfile-maze-runner bundle install

Then Maze Runner can be run by running:

$ BUNDLE_GEMFILE=Gemfile-maze-runner [other env variables] bundle exec maze-runner [feature files]

TESTING.md has been updated with these instructions