[master] Fix for bug #678: Do not "Bind" docker "To" containerd.

[mihao]

Because of the BindTo directive, Docker is permanently stopped by systemd when containerd is temporarily killed and restarted. docker/for-linux#678

[thaJeztah]

From the systemd documentation https://www.freedesktop.org/software/systemd/man/systemd.unit.html#BindsTo=

BindsTo=

Configures requirement dependencies, very similar in style to Requires=. However,
this dependency type is stronger: in addition to the effect of Requires= it
declares that if the unit bound to is stopped, this unit will be stopped too.
This means a unit bound to another unit that suddenly enters inactive state will
be stopped too. Units can suddenly, unexpectedly enter inactive state for different
reasons: the main process of a service unit might terminate on its own choice,
the backing device of a device unit might be unplugged or the mount point of
a mount unit might be unmounted without involvement of the system and service manager.

When used in conjunction with After= on the same unit the behaviour of BindsTo=
is even stronger. In this case, the unit bound to strictly has to be in active
state for this unit to also be in active state. This not only means a unit bound
to another unit that suddenly enters inactive state, but also one that is bound
to another unit that gets skipped due to a failed condition check (such as
ConditionPathExists=, ConditionPathIsSymbolicLink=, … — see below) will be
stopped, should it be running. Hence, in many cases it is best to combine BindsTo=
with After=.

When BindsTo=b.service is used on a.service, this dependency will show as
BoundBy=a.service in property listing of b.service. BoundBy= dependency
cannot be specified directly.

[thaJeztah]

I see that the commit does not have a DCO sign-off (the DCO check was not running when this PR was opened)

@mihao could you rebase this PR, and amend the commit message to have a Signed-off-by line? https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work