Handle null completions with a default callback

Credits to thaJeztah

Signed-off-by: Harald Albers <github@albersweb.de>

cli/command/container/completion.go

@@ -140,6 +140,7 @@ func addCompletions(cmd *cobra.Command, dockerCLI completion.APIClientProvider)
 	_ = cmd.RegisterFlagCompletionFunc("volumes-from", completion.ContainerNames(dockerCLI, true))
 }
 
+// completeCgroupns implements shell completion for the `--cgroupns` option of `run` and `create`.
 func completeCgroupns() completion.ValidArgsFn {
 	return completion.FromList(string(container.CgroupnsModeHost), string(container.CgroupnsModePrivate))
 }

cli/command/container/completion_test.go

@@ -55,7 +55,6 @@ func TestCompletePid(t *testing.T) {
 	}
 
 	for _, tc := range tests {
-		tc := tc
 		t.Run(tc.toComplete, func(t *testing.T) {
 			cli := test.NewFakeCli(&fakeClient{
 				containerListFunc: tc.containerListFunc,
@@ -74,7 +73,7 @@ func TestCompleteRestartPolicies(t *testing.T) {
 	assert.Check(t, is.DeepEqual(values, expected))
 }
 
-func TestCompleteSecurityOpts(t *testing.T) {
+func TestCompleteSecurityOpt(t *testing.T) {
 	tests := []struct {
 		toComplete          string
 		expectedCompletions []string
@@ -119,7 +118,6 @@ func TestCompleteSecurityOpts(t *testing.T) {
 	}
 
 	for _, tc := range tests {
-		tc := tc
 		t.Run(tc.toComplete, func(t *testing.T) {
 			completions, directive := completeSecurityOpt(nil, nil, tc.toComplete)
 			assert.Check(t, is.DeepEqual(completions, tc.expectedCompletions))