Use consistent casing between FROM and AS (#17431)

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

docker/Dockerfile

@@ -27,7 +27,7 @@ ARG PYTHON_VERSION=3.11
 ###
 # We hardcode the use of Debian bookworm here because this could change upstream
 # and other Dockerfiles used for testing are expecting bookworm.
-FROM docker.io/library/python:${PYTHON_VERSION}-slim-bookworm as requirements
+FROM docker.io/library/python:${PYTHON_VERSION}-slim-bookworm AS requirements
 
 # RUN --mount is specific to buildkit and is documented at
 # https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md#build-mounts-run---mount.
@@ -87,7 +87,7 @@ RUN if [ -z "$TEST_ONLY_IGNORE_POETRY_LOCKFILE" ]; then \
 ###
 ### Stage 1: builder
 ###
-FROM docker.io/library/python:${PYTHON_VERSION}-slim-bookworm as builder
+FROM docker.io/library/python:${PYTHON_VERSION}-slim-bookworm AS builder
 
 # install the OS build deps
 RUN \

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 \