No a process running in docker or on your host is going to use the same amount of RAM. There is no RAM dedicated to a running container on Linux--it's just a process that's flagged with some metadata to tell the kernel that it should be isolated in certain ways, like with a unique root filesystem.

Loading in the whole container though does increase the necessary RAM — with nix, two versions of the same executable can link to the same hash, but it is unlikely to be the case with Docker.

Your kernel is fine at managing the container root filesystem and paging in and out parts as necessary. It's not just loading gigabytes of root fs into memory and calling it good.

Files still take up space on disk.

Paging still has a cost.