Add path to soffice so the unoconv export/import works

pica-etherpad/entrypoint.sh

@@ -94,6 +94,7 @@ cat <<- EOF > /opt/etherpad-lite/settings.json
 	"chatAndUsers": false
   },
   "allowUnknownFileEnds": false,
+  "soffice": "/usr/bin/soffice",
 EOF
 
 if [ $ETHERPAD_ADMIN_PASSWORD ]; then