Fix handling of command in images

Currently we are dropping the command entry from the create
line and using the image Cmd.  This change will only use the
image Cmd if the user did not specify a Cmd.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

Closes: #823
Approved by: umohnani8

libpod/container_internal.go

@@ -243,7 +243,9 @@ func (c *Container) setupStorage(ctx context.Context) error {
 
 	// Set the default Entrypoint and Command
 	c.config.Entrypoint = containerInfo.Config.Config.Entrypoint
-	c.config.Command = containerInfo.Config.Config.Cmd
+	if len(c.config.Command) == 0 {
+		c.config.Command = containerInfo.Config.Config.Cmd
+	}
 
 	artifacts := filepath.Join(c.config.StaticDir, artifactsDir)
 	if err := os.MkdirAll(artifacts, 0755); err != nil {