According to Docker upstream release notes [1] MountFlags should be
empty.
1. https://docs.docker.com/engine/release-notes/#18091
"Important notes about this release
In Docker versions prior to 18.09, containerd was managed by the Docker
engine daemon. In Docker Engine 18.09, containerd is managed by systemd.
Since containerd is managed by systemd, any custom configuration to the
docker.service systemd configuration which changes mount settings (for
example, MountFlags=slave) breaks interactions between the Docker Engine
daemon and containerd, and you will not be able to start containers.
Run the following command to get the current value of the MountFlags
property for the docker.service:
sudo systemctl show --property=MountFlags docker.service
MountFlags=
Update your configuration if this command prints a non-empty value for
MountFlags, and restart the docker service."
Closes-bug: #1833835
Change-Id: I4f4cbb09df752d00073a606463c62f0a6ca6c067
35941738d5