registry: warn of DOCKER_AUTH_CONFIG usage in login and logout

Signed-off-by: Alano Terblanche <18033717+Benehiko@users.noreply.github.com>

cli/command/registry/login.go

@@ -110,6 +110,9 @@ func runLogin(ctx context.Context, dockerCLI command.Cli, opts loginOptions) err
 	if err := verifyLoginOptions(dockerCLI, &opts); err != nil {
 		return err
 	}
+
+	maybePrintEnvAuthWarning(dockerCLI)
+
 	var (
 		serverAddress string
 		msg           string

cli/command/registry/logout.go

@@ -36,6 +36,8 @@ func NewLogoutCommand(dockerCli command.Cli) *cobra.Command {
 }
 
 func runLogout(ctx context.Context, dockerCLI command.Cli, serverAddress string) error {
+	maybePrintEnvAuthWarning(dockerCLI)
+
 	var isDefaultRegistry bool
 
 	if serverAddress == "" {

cli/command/registry/warning.go

@@ -0,0 +1,18 @@
+package registry
+
+import (
+	"os"
+
+	"github.com/docker/cli/cli/command"
+	"github.com/docker/cli/cli/config/configfile"
+	"github.com/docker/cli/internal/tui"
+)
+
+// maybePrintEnvAuthWarning if the `DOCKER_AUTH_CONFIG` environment variable is
+// set this function will output a warning to stdErr
+func maybePrintEnvAuthWarning(out command.Streams) {
+	if os.Getenv(configfile.DockerEnvConfigKey) != "" {
+		tui.NewOutput(out.Err()).
+			PrintWarning("%[1]s is set and takes precedence.\nUnset %[1]s to restore the CLI auth behaviour.\n", configfile.DockerEnvConfigKey)
+	}
+}