diff --git a/scripts/custom-docker-build b/scripts/custom-docker-build
index 4493eb4..fbb7c34 100755
--- a/scripts/custom-docker-build
+++ b/scripts/custom-docker-build
@@ -121,6 +121,12 @@ function print_terraform_args() {
 function parse_arguments() {
     read base
     read base_version
+
+    # Lock Ruby to Debian stretch
+    case "$base" in
+        ruby) base_version="$base_version-stretch" ;;
+    esac
+
     printf -- "-f Dockerfile.custom " "$base"
     printf -- "--build-arg CUSTOM_IMAGE_NAME=%s " "$base"
     printf -- "--build-arg CUSTOM_IMAGE_VERSION=%s " "$base_version"