Add docker support

[udenr]

This PR adds Docker support for the scripts so that they can be used on (non‑Linux) machines without having to install all the required dependencies locally.
I performed a quick test converting a short recording to a video and the that succeeded without any issues.

During the initial setup I ran into a few dependency problems, so I added more packages to the Dockerfile than were strictly needed. I have since commented those out, but left them in place in case any of the optional command‑line flags or specific configurations require them.

Usage (as described in the README):

docker compose build
docker compose run --rm bbb-render ./download.py <presentation_url> data/presentation
docker compose run --rm bbb-render ./make-xges.py data/presentation data/project.xges
docker compose run --rm bbb-render ges-launch-1.0 --load data/project.xges -o data/output.mp4