forward local ports in a docker container to the docker host (host.docker.internal)

docker-entrypoint.sh

#!/usr/bin/env bash
main(){
  set -eo pipefail

  # sysctl is a readonly filesystem in containers, so this must be set at run;
  #  [[ "$OSTYPE" =~ darwin|macos* ]] || docker_flags+=("--add-host host.docker.internal:host-gateway")
  #  docker run -it "${docker_flags[@]}" --cap-add=NET_ADMIN  --sysctl net.ipv4.conf.all.route_localnet=1 ...
  sysctl -w net.ipv4.conf.all.route_localnet=1

  docker_host_ip=$(getent ahostsv4 host.docker.internal | head -n1 | awk '{print $1}')
  port_forward_start=8000
  port_forward_end=8999

  iptables -t nat -A OUTPUT -p tcp -m tcp -j DNAT \
      --dport $port_forward_start:$port_forward_end  \
      --to-destination "$docker_host_ip:$port_forward_start-$port_forward_end"
  iptables -t nat -A POSTROUTING -j MASQUERADE

  "$@"
}

main "$@"