[docker/cli] flaky test: TestPromptExitCode/plugin_upgrade, estPromptExitCode/install

### ROOT CAUSE The test failure occurs because the Docker CLI is attempting to push a plugin over HTTPS, but the local registry is configured to use HTTP. This mismatch causes the server to respond with HTTP, which the HTTPS client doesn't accept, leading to the error. ### CODE FIX To resolve the issue, ensure the local registry is started with HTTPS enabled in the test setup. Modify the test configuration to use HTTPS for the registry endpoint and configure the Docker daemon to communicate over HTTPS.

// In the test setup, start the registry with HTTPS
// Example:
// registry := StartRegistryWithHTTPS()
// Then, configure Docker daemon to use HTTPS
// Example:
// docker.Config.Set("registry", "https://registry:5000")


This change ensures that both the Docker CLI and the registry communicate securely over HTTPS, preventing the protocol mismatch error.