Fix --attach flag being silently ignored without a message argument

When running 'docker-agent run --attach image.png' without a message,
readInitialMessage returns nil, so WithFirstMessage was never called.
This caused SendFirstMessage to return nil immediately, silently
discarding the attachment.

Synthesize an empty default first message when --attach is provided
without an explicit message, so SendFirstMessage proceeds to process
the attachment. The downstream CreateUserMessageWithAttachment already
handles empty text by substituting a default prompt.

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

Author: rumpl

cmd/root/run.go

@@ -485,6 +485,11 @@ func (f *runExecFlags) buildAppOpts(args []string) ([]app.Opt, error) {
 	var opts []app.Opt
 	if firstMessage != nil {
 		opts = append(opts, app.WithFirstMessage(*firstMessage))
+	} else if f.attachmentPath != "" {
+		// When --attach is used without an explicit message, provide a default
+		// so that SendFirstMessage processes the attachment.
+		defaultMsg := ""
+		opts = append(opts, app.WithFirstMessage(defaultMsg))
 	}
 	if len(args) > 2 {
 		opts = append(opts, app.WithQueuedMessages(args[2:]))