Review feedback on #6539: give users an explicit opt-out from the IPv6 listener even when the host reports IPv6. The issue reporter (#4596) requested such an env var, and the pattern matches the existing ENABLE_DNS_CACHING IPv6 toggle. Auto-detection behavior is unchanged when the var is unset. Validated in containers: override wins, dual-stack preserved by default, IPv4-only fallback on IPv6-less hosts.