Disable checkout function for (official) docker containers

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

pihole

@@ -413,6 +413,9 @@ tailFunc() {
 }
 
 piholeCheckoutFunc() {
+  if [ -n "${DOCKER_VERSION}" ]; then
+    unsupportedFunc
+  else
     if [[ "$2" == "-h" ]] || [[ "$2" == "--help" ]]; then
       echo "Usage: pihole checkout [repo] [branch]
   Example: 'pihole checkout master' or 'pihole checkout core dev'
@@ -433,6 +436,7 @@ Branches:
     source "${PI_HOLE_SCRIPT_DIR}"/piholeCheckout.sh
     shift
     checkout "$@"
+  fi
 }
 
 tricorderFunc() {