healthcheck: Use IPv4 loopback address instead of localhost

Access to health check resources such as /-/liveness
is restricted to IPs specified in gitlab.monitoring.ip_whitelist
(`GITLAB_MONITORING_IP_WHITELIST`).
The name `localhost` is solved to IPv6 loopback address (::1)
that is not listed in the whitelist by default.

Possible alternate designs:
- Add IPv6 loopback to whitelist
- Disable IPv6 for gitlab container by specifying `net.ipv6.conf.all.disable_ipv6=1`
  in docker-compose.yml for example
  See #2766 (comment)

assets/runtime/functions

@@ -1864,7 +1864,7 @@ generate_healthcheck_script() {
   fi
 cat > /usr/local/sbin/healthcheck <<EOF
 #!/bin/bash
-url=${HEALTHCHECK_PROTOCOL}://localhost${GITLAB_RELATIVE_URL_ROOT}/-/liveness
+url=${HEALTHCHECK_PROTOCOL}://127.0.0.1${GITLAB_RELATIVE_URL_ROOT}/-/liveness
 options=( '--insecure' '--silent' )
 curl "\${options[@]}" \$url
 [[ "\$(curl \${options[@]} -o /dev/null -I -w '%{http_code}' \$url)" == "200" ]]