doc/userguide: ubuntu: install software-properties-common

This package likely needs to be installed when starting with an Ubuntu
container or other minimal Ubuntu install.

Ticket: #5616

doc/userguide/install.rst

@@ -122,6 +122,7 @@ For Ubuntu, the OISF maintains a PPA ``suricata-stable`` that always contains th
 
 To use it::
 
+    sudo apt-get install software-properties-common
     sudo add-apt-repository ppa:oisf/suricata-stable
     sudo apt-get update
     sudo apt-get install suricata