Fix: recording rendering crash for presentations containing polls

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We've had a problem getting a successful rendering of a meeting recording in which presentation contained a number of polls. It was failing due to some missing Python3 dependencies. The following (executed on the recordings container) fixed the issue:

apt install python3-attr python3-cairo python3-gi python3-gi-cairo gir1.2-pangocairo-1.0

So these dependencies can and should be added to the recordings Dockerfile!

Note this problem occurs both with the current stable and dev versions of the Docker containers.

Hope this helps someone!

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Actually, I've just submitted a pull request: https://github.com/bigbluebutton/docker/pull/205