message: Print a warning if the hostname or IP is invalid
  See
diff: @@ -402,7 +402,6 @@ install_shadowbox() {
  readonly SB_IMAGE=${SB_IMAGE:-quay.io/outline/shadowbox:stable}
  log_for_sentry "Setting PUBLIC_HOSTNAME"
  - # TODO(fortuna): Make sure this is IPv4
  PUBLIC_HOSTNAME=${FLAGS_HOSTNAME:-${SB_PUBLIC_IP:-$(curl -4s https://ipinfo.io/ip)}}
  if [[ -z $PUBLIC_HOSTNAME ]]; then
  @@ -410,6 +409,11 @@ install_shadowbox() {
  log_error "$MSG"
  log_for_sentry "$MSG"
  exit 1
  + elif ! getent ahosts $PUBLIC_HOSTNAME > /dev/null; then
  + local readonly MSG="Invalid IP or domain name: \"$PUBLIC_HOSTNAME\""
  + log_error "$MSG"
  + log_for_sentry "$MSG"
  + exit 1
  fi
  # If $ACCESS_CONFIG already exists, copy it to backup then clear it.
  