I don't want to introduce a dependency to this /.docker-id file. How about instead we generate a unique devmapper name for a given /var/lib/docker directory. We could simply use the directory's inode (and maybe something to differentiate the mount). This seems much cleaner and generic. For example it 1) also works on multiple instances of docker within the host, and 2) if docker runs inside a container but *on the same /var/lib/docker* (via bind-mount), that will be detected and the same device-mapper will be used.