Add an option to disable including `libavcodec.so` in the firefox container.
Right now, the firefox container explcitly includes the "best" libavcodec shared library (dlopen()ed by firefox), 100% of the time as a concession to usability.
Since this is optional, and there are usage models that do not require video/audio playback, this should be made optional like how PulseAudio currently is, under the rationale that people who can live with omitting it, can gain reduced attack surface.
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