Running coach on docker with JSON output delivers invalid JSON

[ahoiroman]

The generated JSON is invalid:

1. Run doer run sitespeedio/coach:4.5.0 https://www.sitespeed.io -f json
2. Result contains log output and this is invalid JSON:

Result:

Google Chrome 80.0.3987.87 
Mozilla Firefox 73.0
[2020-08-31 14:35:50] INFO: Running tests using Chrome - 1 iteration(s)
[2020-08-31 14:35:50] INFO: Testing url https://www.sitespeed.io iteration 1

{...}

It should be possible to mute the log.