InvenTree: run apt under systemd so postinst can finish (#18029)

1.6.0's postinst walks parent PIDs until PID 1 to collect settings. Under pct exec the chain ends at PID 0, and since InvenTree #12995 turned the loop's [ ] into [[ ]] it no longer breaks out there, so the install hung printing ps and strings errors. systemd-run makes systemd the parent.

ct/inventree.sh

@@ -58,7 +58,9 @@ function update_script() {
 
   msg_info "Updating InvenTree"
   apt_update_safe
-  $STD apt install --only-upgrade inventree -y
+  # postinst loops forever unless its parent chain reaches PID 1 (not the case under pct exec)
+  $STD systemd-run --wait --pipe --quiet --collect --setenv=DEBIAN_FRONTEND=noninteractive \
+    apt-get install --only-upgrade -y inventree
   msg_ok "Updated InvenTree"
   msg_ok "Updated successfully!"
   exit

install/inventree-install.sh

@@ -23,8 +23,10 @@ setup_deb822_repo \
 msg_ok "Set up InvenTree Repository"
 
 msg_info "Installing InvenTree (Patience)"
-export SETUP_NO_CALLS=true
-$STD apt install -y inventree
+# The package's postinst walks parent PIDs until it reaches PID 1; under pct exec the chain ends at 0
+# and since 1.6.0 that loops forever, so apt runs as a child of systemd
+$STD systemd-run --wait --pipe --quiet --collect --setenv=SETUP_NO_CALLS=true --setenv=DEBIAN_FRONTEND=noninteractive \
+  apt-get install -y inventree
 msg_ok "Installed InvenTree"
 
 msg_info "Configuring InvenTree"