Fix for #8777

Now filter name is trimmed and lowercased before evaluation for case
insensitive and whitespace trimemd check.

Signed-off-by: Oh Jinkyun <tintypemolly@gmail.com>

api/client/commands.go

@@ -47,6 +47,10 @@ const (
 	tarHeaderSize = 512
 )
 
+var (
+	acceptedImageFilterTags = map[string]struct{}{"dangling": {}}
+)
+
 func (cli *DockerCli) CmdHelp(args ...string) error {
 	if len(args) > 1 {
 		method, exists := cli.getMethod(args[:2]...)
@@ -1336,6 +1340,12 @@ func (cli *DockerCli) CmdImages(args ...string) error {
 		}
 	}
 
+	for name := range imageFilterArgs {
+		if _, ok := acceptedImageFilterTags[name]; !ok {
+			return fmt.Errorf("Invalid filter '%s'", name)
+		}
+	}
+
 	matchName := cmd.Arg(0)
 	// FIXME: --viz and --tree are deprecated. Remove them in a future version.
 	if *flViz || *flTree {