Add the following to your NixOS configuration:
virtualisation.docker.enable = true;
and
sudo nixos-rebuild switch
You may have to restart at this point. Once you have Docker installed and running,
sudo docker run -d --restart=always --name=dropbox \
-v ~/Dropbox:/dbox/Dropbox \
janeczku/dropbox
and follow the instructions here: https://hub.docker.com/r/janeczku/dropbox/