Commit ed13c3abfb242905ec012e8255dc6f26dcf122f6 added flags
for Docker Content Trust. Depending on the `verify` boolean,
the message is "Skip image verification", or "Skip image signing".
"Signing" is intended for `docker push` / `docker plugin push`.
During the migration to Cobra, this boolean got flipped for
`docker push` (9640e3a4514f96a890310757a09fd77a3c70e931),
causing `docker push` to show the incorrect flag description.
This patch changes the flags to use the correct description
for `docker push`, and `docker plugin push`.
To prevent this confusion in future, the boolean argument
is removed, and a `AddTrustSigningFlags()` function is added.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
