resolve ip for host.docker.internal DNS entry

resolve-docker-host-ip.sh

# How to use:
# add to Dockerfile, otherwise add to entrypoint script. For example:
# RUN mkdir -p /bin/docker-entrypoint/ \
#  && cp /tmp/scripts/docker-entrypoint/* /bin/docker-entrypoint/ \
#  && chmod +x -R /bin/docker-entrypoint/ \
# ;

# ENTRYPOINT ["/bin/docker-entrypoint/resolve-docker-host-ip.sh", ...]
set -e

HOST_DOMAIN="host.docker.internal"

# check if the host exists - this will fail on linux
if dig ${HOST_DOMAIN} | grep -q 'NXDOMAIN'
then
  # resolve the host IP
  HOST_IP=$(ip route | awk 'NR==1 {print $3}')
  # and write it to the hosts file
  echo "$HOST_IP\t$HOST_DOMAIN" >> /etc/hosts
fi

exec "$@"