geckodriver Docker image support?

[linux-china]

I want to run Scout inside container, and it's possible to use service supplied by geckodriver Docker image.

For example, if GECKODRIVER_URL or SCOUT_URL supplied, such as 128.0.2.0.1:4444, and Scout could use remote service .

[maxmindlin]

Scout has a docker image available that is pushed on every tag update which could help you here. This image includes Firefox and gecko driver as well.

As far as allowing for remote geckodriver urls, Im open to a PR. It would be similar to other available ENV vars, and probably named SCOUT_GECKODRIVER_URL.