videos can have their own "containers" too, for instance in AV1 the stream is stored inside of an obu, which is wrapped in a external container. (such as matroska) if you really wanted to you could (and can) put images into containers too, PNGs in a matroska are actually pretty useful way for transfering PNG sequences.

you can also with a simple mod on a older commit of ffmpeg (the commit that added animated jxl broke this method and I haven't gotten around to fixing it) by simply adding jxl 4cc to mux JXL sequences into MKV.