Build FFmpeg on manylinux image

[andfoy]

This PR adds ffmpeg to the manylinux Docker image

[seemethere]

Looks like there's an issue with the build, we might be missing a dependency:

#47 56.78 + ./configure --disable-doc --disable-openssl --enable-avresample --enable-gnutls --enable-hardcoded-tables --enable-libfreetype --enable-libopenh264 --enable-pic --enable-pthreads --enable-shared --disable-static --enable-version3 --enable-zlib --enable-libmp3lame
#47 64.78 ERROR: libmp3lame >= 3.98.3 not found

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