* Use docker-compose>=1.18 and minimal docker-compose config version to 2.3
* Starts ui-tests in Firefox docker container
* Let's the Firefox docker-container hit directly nginx
* makes use of https://github.com/jrbenny35/selenium-firefox/ firefox+selenium image
* makes use of /user-media/ serving directly via nginx for XPI installs
* Adds a helper that creates an installable add-on
* Update `firefox_options` fixture with all options needed to install add-ons with a test signing signature
Fixes#7270Fixes#2488
* Updated config to use circleci as well as configure ui-tests to run within docker
* Removed hacky certifi installs
* Update requirements to master versions
Fixes#4697 (though not only that)