Pin Redis image for entrypoint env tests

[sjmiller609]

Summary

• replace mutable bitnami/redis:latest in entrypoint env tests with the legacy Bitnami Redis 7.2.5 Debian image
• share the pinned image ref across Cloud Hypervisor and QEMU variants

Why

Main was failing on deft-kernel-dev with misleading vsock errors in TestEntrypointEnvVars and TestQEMUEntrypointEnvVars. The VM logs showed the guest agent did become ready, but the current bitnami/redis:latest image exited immediately with:

Fatal error, can't open config file '/opt/bitnami/redis/etc/redis.conf': Permission denied

That powered off the VM, so later guest exec attempts reported missing/no-device vsock.

Validation

• On deft-kernel-dev, after mirroring docker.io/bitnamilegacy/redis:7.2.5-debian-12-r0 into the local CI registry:
  go test -tags containers_image_openpgp -run "Test(EntrypointEnvVars|QEMUEntrypointEnvVars)$" -count=1 -v -timeout=8m ./lib/instances
• Both tests passed in 6.826s.

---

Note

Low Risk
Test and CI prewarm image list only; no production runtime or auth logic changes.

Overview
Replaces the floating docker.io/bitnami/redis:latest reference with a pinned docker.io/bitnamilegacy/redis:7.2.5-debian-12-r0 so entrypoint env integration tests stop failing when upstream latest breaks in the guest (e.g. Redis exiting on config permission errors and surfacing as vsock/exec failures).

Adds a shared redisEntrypointEnvImage constant in manager_test.go and uses it in TestEntrypointEnvVars and TestQEMUEntrypointEnvVars for both image pull and instance create. cmd/test-prewarm mirrors the same tag in defaultImages, with a comment to keep it aligned with the test constant.

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[IlyaasK]

seems that redis being latest is not mission critical, so lgtm