Fix FromAsCasing

Docker complains about FROM ${NGINX_FROM_IMAGE} as builder because `'as' and 'FROM' keywords' casing do not match` [info](https://docs.docker.com/reference/build-checks/from-as-casing/).

Super simple fix. Absolutely a minor fix, but it's one less warning.

modules/Dockerfile

 ARG NGINX_FROM_IMAGE=nginx:mainline
-FROM ${NGINX_FROM_IMAGE} as builder
+FROM ${NGINX_FROM_IMAGE} AS builder
 
 ARG ENABLED_MODULES