If docker version env var is set, store it in /etc/pihole/versions file for later use

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

advanced/Scripts/updatecheck.sh

@@ -84,4 +84,8 @@ else
     FTL_VERSION="$(pihole-FTL version)"
     addOrEditKeyValPair "${VERSION_FILE}" "FTL_VERSION" "${FTL_VERSION}"
 
+    if [[ "${PIHOLE_DOCKER_TAG}" ]]; then
+        addOrEditKeyValPair "${VERSION_FILE}" "DOCKER_VERSION" "${PIHOLE_DOCKER_TAG}"
+    fi
+
 fi