I write, as it will be useful to people who play with apparmor
I also got this problem on my Ubuntu machine. This happened because I ran the aa-genprof docker command to scan the docker command to create the apparmor profile. Therefore, the scanned process created the file usr.bin.docker in the directory /etc/apparmor.d , which added some permissions for the docker command. Thus, after deleting this file and rebooting, the Docker machine works fine again.
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