Work around bad hostname detection during postfix package install. (#9878)

docker/prod/setup/preinstall-on-prem.sh

@@ -2,6 +2,18 @@
 set -e
 set -o pipefail
 
+# postfix.postinst tries to generate a hostname based on /etc/resolv.conf, which
+# gets copied in to the docker environment from the host system. On systems
+# that are not on a network with a domain, this will result in a failed install.
+#
+# See https://salsa.debian.org/postfix-team/postfix-dev/-/blob/debian/buster-updates/debian/postfix.postinst#L40
+if [ -f /run/.containerenv -o -f /.dockerenv ]; then
+	mv /bin/hostname /bin/x-hostname
+	echo openproject.local > /etc/hostname
+	apt-get install -y postfix
+	mv /bin/x-hostname /bin/hostname
+fi
+
 apt-get install -y  \
 	memcached \
 	postfix \