Use consistent casing between FROM and AS (#17431)

Signed-off-by: Joe Groocock <me@frebib.net>

docker/Dockerfile-dhvirtualenv

@@ -24,7 +24,7 @@ ARG distro=""
 # https://launchpad.net/~jyrki-pulliainen/+archive/ubuntu/dh-virtualenv, but
 # it's not obviously easier to use that than to build our own.)
 
-FROM docker.io/library/${distro} as builder
+FROM docker.io/library/${distro} AS builder
 
 RUN apt-get update -qq -o Acquire::Languages=none
 RUN env DEBIAN_FRONTEND=noninteractive apt-get install \