Validation of Mounts was only performed on container _creation_, not on
container _start_. As a result, if the host-path no longer existed
when the container was started, a directory was created in the given
location.
This is the wrong behavior, because when using the `Mounts` API, host paths
should never be created, and an error should be produced instead.
This patch adds a validation step on container start, and produces an
error if the host path is not found.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
7cb96ba308