Use linuxserver/ffmpeg as a base image

[lizardfish0]

As discussed in #49.

Tried to make as few changes as possible to the existing environment variables so that this doesn't break user's setups.

• Left the SMA_FFMPEG_URL option in place for anyone who was utilizing it. You'll still have all the environment variables and junk from the linuxserver/ffmpeg image, but if for some reason you want a specific binary you can do that.
• Removed SMA_USE_REPO as this no longer makes much sense.
• Removed some apt lines and venv setup from the Dockerfile as it seems like this is covered by s6.
• Removed SMA_HWACCEL as hardware acceleration libs are included in the ffmpeg container
• Removed fontconfig from the r6 package installations since it is built from source in the ffmpeg container

This change now leaves build and develop in a slightly weird place. linuxserver/sonarr:develop is built on Alpine instead of Jammy (no discussion in their repo about consolidating the two). I haven't tried layering the images but I imagine there will be issues there.

build now has a slight divergence in config options. SMA_HWACCEL is no longer needed, but some might be relying on it to install libraries required by their custom ffmpeg binaries.

My vote would be to deprecate SMA_HWACCEL from build and have people use the "hidden" SMA_EXTRA_PACKAGES instead. Then, they can specify which image they want to compile from: jrottenberg/ffmpeg or linuxserver/ffmpeg using ffmpeg_source/ffmpeg_tag, and install any extra packages they need using that var.

[lizardfish0]

Actually, I think the cleanest option might be to combine master and build. Let me know what you think of #51.