OK, but then you should understand that snaps are extremely restrictive. There is a lot of things they do not let you do. Complain to Ubuntu or Debian about that one 
It’s to save people with little experience of DBs a world of pain. Again, be aware of what snaps are allowed/not allowed to do. Almost certainly a snap Rocket would not be able to talk to a local Mongo. Because of snaps structure, not anything else.
Hmm. It seems that it is actually snaps not giving you the flexibility as docker will do this.
You should consider that your view is likely because you don’t understand enough about either docker, or snaps, or both.
Snaps are meant to be simple and safe. As such they are very tightly controlled about what they can and cannot do. As above.
Docker is much more flexible for a huge variety of reasons. But the result is something that is also much more open to abuse. See say bridged vs host networking. Etc.
FYI for various reasons I run my own systems with a native Mongo and a dockererised Rocket. Easy.
If that is the scenario you want, use docker as it will not be available in snaps.