fix(static): ensure that FRANKENPHP_VERSION is defined in child Docker images

static-builder.Dockerfile

@@ -2,7 +2,11 @@
 FROM golang-base
 
 ARG FRANKENPHP_VERSION=''
+ENV FRANKENPHP_VERSION=${FRANKENPHP_VERSION}
+
 ARG PHP_VERSION=''
+ENV PHP_VERSION=${PHP_VERSION}
+
 ARG PHP_EXTENSIONS=''
 ARG PHP_EXTENSION_LIBS=''
 ARG CLEAN=''