Bump to 4.3.1 and fix some bugs in Docker image

[joshuaboniface]

Replaces #41

[joshuaboniface]

This does still fail. Despite the configuration for this not having changed, It's unable to find the nvidia headers when running the ./configure step:

ERROR: cuvid requested, but not all dependencies are satisfied: ffnvcodec

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
Include the log file "ffbuild/config.log" produced by configure as this will help
solve the problem.

I don't see a good reason why, and the sheer amount of time it takes to debug this is annoying.

At the moment, I don't believe we actually use this Dockerfile for anything, as our main Docker images are installing jellyfin-ffmpeg via the Debian packages instead of pulling from an FFfmpeg image. I would like that to change in the future, but I don't want to block the 10.6.0 release or 4.3.1 ffmpeg release on this.