Fix casing on Dockerfile

Closes: https://github.com/netbootxyz/netboot.xyz/issues/1532

Dockerfile

@@ -1,6 +1,6 @@
 ARG NBXYZ_OVERRIDES=default
 
-FROM ghcr.io/netbootxyz/builder:latest as builder
+FROM ghcr.io/netbootxyz/builder:latest AS builder
 
 # repo for build
 COPY . /ansible
@@ -11,7 +11,7 @@ ENV EXTRA_VARS=""
 FROM builder AS netbootxyz-production
 ENV EXTRA_VARS="--extra-vars @script/netbootxyz-overrides.yml"
 
-FROM netbootxyz-${NBXYZ_OVERRIDES} as final
+FROM netbootxyz-${NBXYZ_OVERRIDES} AS final
 RUN \
   echo "**** running ansible ****" && \
   cd /ansible && \