This PR enhances the production Dockerfile by converting from installing the version ffmpeg that is available in the Debian repos, to using a custom built versions. This has the following advantages:

• Image size reduction: the current Dockerfile was optimized in Dockerfile rewrite based on Ruby image with performance optimizations and size reduction, dedicated Streaming image #26850 through a number of methods including separating the Core Image and Streaming image, removing Node from the resulting image. The 4.2 image is approximately 1.5GB in size while the current 4.3 image is just under 1GB. When ffmpeg is installed from the Debian repo, it includes a number of dependencies to support file formats that are not used or explicitly not desired in Mastodon, including the unused X11 server. Build libvips from source in Dockerfile #30571 took the first step in compiling libvips from source to eliminate this dependency. The size of the resulting Docker image built by this PR under 700MB!
• Controlled version adoption: ffmpeg version control should be considered as essential as Ruby, Node, Postgres, or any other Mastodon dependency given the importance of media processing. The Debian repos are notoriously slow in adoption of upgrades, for example ffmpeg is currently at 5.1.4 when 7.0.1 is available.
• Optimized library selection: Compiling these packages from source allows us to use only the libraries that are necessary for Mastodon. Ultimately I would like to become more restrictive about what formats are compiled as supported in ffmpeg as most are not needed by Mastodon.

ffmpeg compile/library settings

• Versions can be controlled in a similar method as Ruby/Node through Docker build arguments, to enable easier local testing and customized version deployments. [--build-arg FFMPEG_VERSION="7.0.1"]
• Source code is downloaded directly from ffmpeg.org.
• Building documentation and direct network support are disabled. (Full list of compile options available at https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu)
• libx264 is used for H.264 video encoding
• libx265 is used for HEVC video encoding
• libvpx is used for VP8/VP9 video encoding/decoding
• libopus is used for Opus audio encoding/decoding
• libdav1d is used for AV1 decoding
• libmp3lame is used for MP3 encoding
• libvorbis is used for Vorbis audio encoding/decoding
• libwebp is used for WebP encoding
• libsnappy is used for compression

$ ffmpeg -version
ffmpeg version 7.0.1 Copyright (c) 2000-2024 the FFmpeg developers
built with gcc 12 (Debian 12.2.0-14)
configuration: --prefix=/usr/local/ffmpeg --toolchain=hardened --disable-doc --disable-network --disable-static --disable-debug --disable-ffplay --disable-devices --enable-gpl --enable-version3 --enable-shared --enable-ffmpeg --enable-ffprobe --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-libsnappy --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libwebp
libavutil      59.  8.100 / 59.  8.100
libavcodec     61.  3.100 / 61.  3.100
libavformat    61.  1.100 / 61.  1.100
libavdevice    61.  1.100 / 61.  1.100
libavfilter    10.  1.100 / 10.  1.100
libswscale      8.  1.100 /  8.  1.100
libswresample   5.  1.100 /  5.  1.100
libpostproc    58.  1.100 / 58.  1.100