Dockerfile: Stop varying with the random container hostname.

Author: alexmv

Dockerfile

@@ -62,7 +62,7 @@ RUN \
  mv zulip-server-docker zulip && \
  cp -rf /root/custom_zulip/* /root/zulip && \
  rm -rf /root/custom_zulip && \
- /root/zulip/scripts/setup/install --hostname="$(hostname)" --email="docker-zulip" \
+ /root/zulip/scripts/setup/install --hostname="docker-zulip.local" --email="docker-zulip" \
  --puppet-classes="zulip::profile::docker" --postgresql-version=14 && \
  rm -f /etc/zulip/zulip-secrets.conf /etc/zulip/settings.py && \
  apt-get -qq autoremove --purge -y && \