e2e/image: add private registry pull/push regression test

[AryanSharma9917]

• fixes: Add e2e test for authorized pull/push #5965

Summary

• add authenticated private-registry service to the e2e compose stack
• add fixture auth config for private-registry:5001
• add TestPullPushPrivateRepository to verify unauthenticated pull/push fails and authenticated pull/push succeeds
• add htpasswd test credentials for the private registry

Context

• regression coverage for private repository auth behavior
• refs Unable to pull image from private repository after updating docker-ce-cli to 28.0.3 #5963, Add e2e test for authorized pull/push #5965

Notes

• change is limited to e2e test infrastructure and tests; no production CLI behavior changes

[AryanSharma9917]

Hi @vvoland , could someone please help approve the workflow runs for this PR and take a quick review pass when possible? Thanks in advance!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[vvoland]

Thanks @AryanSharma9917!

I approved the workflows

[thaJeztah]

=== Failed
=== FAIL: e2e/image TestPullPushPrivateRepository (1.57s)
    private_test.go:37: assertion failed: expression is false: strings.Contains(output, "requested access to the resource is denied") || strings.Contains(output, "no basic auth credentials") || strings.Contains(output, "unauthorized") || strings.Contains(output, "authentication required"): The push refers to repository [private-registry:5001/private/alpine]
        Get "http://private-registry:5001/v2/": dial tcp: lookup private-registry on 127.0.0.11:53: server misbehaving
        
    private_test.go:43: assertion failed: 
        Command:  docker push private-registry:5001/private/alpine:test-private-pull-push
        ExitCode: 1
        Error:    exit status 1
        Stdout:   The push refers to repository [private-registry:5001/private/alpine]
        
        Stderr:   Get "http://private-registry:5001/v2/": dial tcp: lookup private-registry on 127.0.0.11:53: server misbehaving
        
        
        Failures:
        ExitCode was 1 expected 0
        Expected no error