[docs]: Add information about set up docker image for video recording

[miherlosev]

The official TestCafe's docker image doesn't contain the ffmpeg library due to license restrictions.
So, the video recording feature is not working in the docker image out of box.
To make it work, need to create a new image based on the TestCafe docker image with the installed ffmpeg library.
First, create a new docker file with the following content:
For example, I've used the following Dockerfile:

FROM testcafe/testcafe
USER root
RUN apk add --no-cache ffmpeg
USER user

When build a new image:

docker build -t tc_with_ffmpeg .
docker run -it -v ${PWD}:/tests -w /tests tc_with_ffmpeg chromium test.js --video videos

The built image support the video recording during test execution.

[titerman]

Closed in favour of #7153