Set up an ongoing Firefox 57 (Quantum, desktop) set of test runs

[stephendonner]

To be able to compare to Quantum's performance in Firefox 57 to current release + Nightly, we should add a Firefox 57 (starting with desktop) run, alongside our current ones.

We (WebPagetest, wptagent, and webpagetest-api) don't yet have browser versioning set up, but I know Browsertime/sitespeed.io has latest/latest-n versioning set up via Docker:

https://www.sitespeed.io/documentation/sitespeed.io/browsers/

Docker "base" image for the browsers: https://hub.docker.com/r/sitespeedio/webbrowsers/dockerfile

Dockerfile for above Docker image/container: https://github.com/sitespeedio/docker-browsers/blob/d250e26747f1b72e08d1ddb07694f2ab603624a8/Dockerfile#L7

[stephendonner]

See catchpoint/WebPageTest.agent#124 and its commits.

Current thoughts:

• Use tee or similar to dynamically replace/write out a browsers.ini file with an entry for the custom, requested build for a given Firefox version
• And the setup for the above might be in:
  https://github.com/WPO-Foundation/wptagent/blob/d5d74a9cfaa67f0f429df05710e707e0af10d21f/wptagent.py#L636-L657

e.g.

firefox_custom_ver_path = '/usr/lib/firefox-${custom_ver}/firefox-${custom_ver}]'
        if 'Firefox' not in browsers and os.path.isfile(firefox_custom_ver_path):
            browsers['Firefox'] = {'exe': firefox_custom_ver_path, 'type': 'Firefox'}
'log_level': 5}

[0] re: ${custom_ver} - used for illustration; needs properly formatted Python 3 strings.