cmd/docker: fix unused-receiver (revive)

cmd/docker/builder_test.go:130:7: unused-receiver: method receiver 'c' is not referenced in method's body, consider removing or renaming it as _ (revive)
    func (c *fakeClient) Ping(_ context.Context) (types.Ping, error) {
          ^

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

cmd/docker/builder_test.go

@@ -127,7 +127,7 @@ type fakeClient struct {
 	client.Client
 }
 
-func (c *fakeClient) Ping(_ context.Context) (types.Ping, error) {
+func (*fakeClient) Ping(context.Context) (types.Ping, error) {
 	return types.Ping{OSType: "linux"}, nil
 }