images: print hint when invoking "docker images" with ambiguous argument

The `docker images` top-level subcommand predates the `docker <object> <verb>`
convention (e.g. `docker image ls`), but accepts a positional argument to
search/filter images by name (globbing). It's common for users to accidentally
mistake these commands, and to use (e.g.) `docker images ls`, expecting
to see all images, but ending up with an empty list because no image named
"ls" was found.

Disallowing these search-terms would be a breaking change, but we can print
and informational message to help the users correct their mistake.

Before this patch:

    docker images ls
    REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

With this patch applied:

    docker images ls
    REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

    No images found matching "ls": did you mean "docker image ls"?

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/list_test.go

@@ -95,3 +95,17 @@ func TestNewListCommandAlias(t *testing.T) {
 	assert.Check(t, cmd.HasAlias("list"))
 	assert.Check(t, !cmd.HasAlias("other"))
 }
+
+func TestNewListCommandAmbiguous(t *testing.T) {
+	cli := test.NewFakeCli(&fakeClient{})
+	cmd := NewImagesCommand(cli)
+	cmd.SetOut(io.Discard)
+
+	// Set the Use field to mimic that the command was called as "docker images",
+	// not "docker image ls".
+	cmd.Use = "images"
+	cmd.SetArgs([]string{"ls"})
+	err := cmd.Execute()
+	assert.NilError(t, err)
+	golden.Assert(t, cli.ErrBuffer().String(), "list-command-ambiguous.golden")
+}