Move mp4 into mkv container, copying bytestream
{{{sh
ffmpeg -i input.mp4 -c copy output.mkv
}}}
Add subtitles to file
{{{sh
ffmpeg -i *.mp4 -i *.srt -c copy out.mkv
}}}
Add cover art to mkv file
{{{sh
ffmpeg -i input.mkv -c copy -attach cover-art.png -metadata:s:t mimetype=image/png output.mkv
}}}
--------------------------------------------------
{{{sh
ffmpeg -i \
-c:v libx264 -map 0:v \
-map 0:a:0? -c:a:0 aac \
-map 0:a:0? -c:a:0 aac \