If ${USER} variable is blank, then populate it with whoami (#5341)

pihole

@@ -544,6 +544,13 @@ if [[ ! $EUID -eq 0 && need_root -eq 1 ]];then
     exit 1
   fi
 fi
+
+# In the case of alpine running in a container, the USER variable appears to be blank
+# which prevents the next trap from working correctly. Set it by running whoami
+if [[ -z ${USER} ]]; then
+  USER=$(whoami)
+fi
+
 # Can also be user pihole for other functions
 if [[ ${USER} != "pihole" && need_root -eq 0 ]];then
   if [[ -x "$(command -v sudo)" ]]; then