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

[AryanSharma9917]

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!