As reported in #11294, the Docker daemon will execute contains it
shouldn't run in the event that a requested tag is present in an image's
ID. This leads to the wrong image being started up silently.
This change reduces the risk of such a collision by using the short ID
iff the actual revOrTag looks like a short ID (not that it necessarily
is).
Signed-off-by: Richard Burnison <rburnison@ebay.com>
