fix flag descriptions for content-trust

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>

command/image/build.go

@@ -108,7 +108,7 @@ func NewBuildCommand(dockerCli *command.DockerCli) *cobra.Command {
 	flags.StringSliceVar(&options.securityOpt, "security-opt", []string{}, "Security options")
 	flags.StringVar(&options.networkMode, "network", "default", "Set the networking mode for the RUN instructions during build")
 
-	command.AddTrustedFlags(flags, true)
+	command.AddTrustVerificationFlags(flags)
 
 	flags.BoolVar(&options.squash, "squash", false, "Squash newly built layers into a single new layer")
 	flags.SetAnnotation("squash", "experimental", nil)

command/image/pull.go

@@ -36,7 +36,7 @@ func NewPullCommand(dockerCli *command.DockerCli) *cobra.Command {
 	flags := cmd.Flags()
 
 	flags.BoolVarP(&opts.all, "all-tags", "a", false, "Download all tagged images in the repository")
-	command.AddTrustedFlags(flags, true)
+	command.AddTrustVerificationFlags(flags)
 
 	return cmd
 }

command/image/push.go

@@ -24,7 +24,7 @@ func NewPushCommand(dockerCli *command.DockerCli) *cobra.Command {
 
 	flags := cmd.Flags()
 
-	command.AddTrustedFlags(flags, true)
+	command.AddTrustSigningFlags(flags)
 
 	return cmd
 }