Some mobile phones can shoot photos in raw by using applications which use Camera2 API. Usually they write raw shots in DNG format and without any compression.
I tried to search the web for free software which can be used directly to convert uncompressed DNG to lossless-compressed DNG, but only found a library named
liblj92, which implements Lossless JPEG compression scheme used in DNG, but if I understood correctly, it doesn't deal with DNG format itself, so it's not the whole thing.
There's also a proprietary converter from Adobe, but this doesn't qualify because I'm searching for free software (free = as in freedom, when I can inspect and modify source code and distribute derivative works under same free license)