gh-38251: .ci/write-dockerfile.sh, docker/Dockerfile: Fix Docker warning FromAsCasing
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes #12345". -->

Newest versions of Docker buildx give a syntax warning
https://github.com/sagemath/sage/actions/runs/9605919331/job/26494464435
?pr=38227#step:13:194
```
WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line
5)
```

### :memo: Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### :hourglass: Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #38251
Reported by: Matthias Köppe
Reviewer(s): Sebastian Oehms, sebastian-oehms

Author: Release Manager

.ci/write-dockerfile.sh

@@ -55,7 +55,7 @@ ADD="ADD $__CHOWN"
 RUN=RUN
 cat <<EOF
 ARG BASE_IMAGE=$(eval echo "${FULL_BASE_IMAGE_AND_TAG}")
-FROM \${BASE_IMAGE} as with-system-packages
+FROM \${BASE_IMAGE} AS with-system-packages
 EOF
 case $SYSTEM in
     debian*|ubuntu*)
@@ -199,7 +199,7 @@ EOF
     *)
         cat <<EOF
 ARG BASE_IMAGE
-FROM \${BASE_IMAGE} as with-system-packages
+FROM \${BASE_IMAGE} AS with-system-packages
 EOF
         INSTALL=$(sage-print-system-package-command $SYSTEM install " ")
         ;;
@@ -268,7 +268,7 @@ fi
 
 cat <<EOF
 
-FROM with-system-packages as bootstrapped
+FROM with-system-packages AS bootstrapped
 #:bootstrapping:
 RUN rm -rf /new /sage/.git
 $ADD Makefile VERSION.txt COPYING.txt condarc.yml README.md bootstrap bootstrap-conda configure.ac sage .homebrew-build-env tox.ini Pipfile.m4 .gitignore /new/
@@ -301,7 +301,7 @@ WORKDIR /sage
 ARG BOOTSTRAP="${BOOTSTRAP-./bootstrap}"
 $RUN sh -x -c "\${BOOTSTRAP}" $ENDRUN $THEN_SAVE_STATUS
 
-FROM bootstrapped as configured
+FROM bootstrapped AS configured
 #:configuring:
 RUN $CHECK_STATUS_THEN mkdir -p logs/pkgs; rm -f config.log; ln -s logs/pkgs/config.log config.log
 ARG CONFIGURE_ARGS="${CONFIGURE_ARGS:---enable-build-as-root}"
@@ -317,7 +317,7 @@ EOF
 fi
 cat <<EOF
 
-FROM configured as with-base-toolchain
+FROM configured AS with-base-toolchain
 # We first compile base-toolchain because otherwise lots of packages are missing their dependency on 'patch'
 ARG NUMPROC=8
 ENV MAKE="make -j\${NUMPROC}"
@@ -327,7 +327,7 @@ ENV SAGE_CHECK_PACKAGES="!cython,!r,!python3,!gap,!cysignals,!linbox,!git,!ppl,!
 #:toolchain:
 $RUN $CHECK_STATUS_THEN make \${USE_MAKEFLAGS} base-toolchain $ENDRUN $THEN_SAVE_STATUS
 
-FROM with-base-toolchain as with-targets-pre
+FROM with-base-toolchain AS with-targets-pre
 ARG NUMPROC=8
 ENV MAKE="make -j\${NUMPROC}"
 ARG USE_MAKEFLAGS="-k V=0"
@@ -337,7 +337,7 @@ ENV SAGE_CHECK_PACKAGES="!cython,!r,!python3,!gap,!cysignals,!linbox,!git,!ppl,!
 ARG TARGETS_PRE="all-sage-local"
 $RUN $CHECK_STATUS_THEN make SAGE_SPKG="sage-spkg -y -o" \${USE_MAKEFLAGS} \${TARGETS_PRE} $ENDRUN $THEN_SAVE_STATUS
 
-FROM with-targets-pre as with-targets
+FROM with-targets-pre AS with-targets
 ARG NUMPROC=8
 ENV MAKE="make -j\${NUMPROC}"
 ARG USE_MAKEFLAGS="-k V=0"
@@ -359,7 +359,7 @@ RUN cd /new && rm -rf .git && \
 ARG TARGETS="build"
 $RUN $CHECK_STATUS_THEN make SAGE_SPKG="sage-spkg -y -o" \${USE_MAKEFLAGS} \${TARGETS} $ENDRUN $THEN_SAVE_STATUS
 
-FROM with-targets as with-targets-optional
+FROM with-targets AS with-targets-optional
 ARG NUMPROC=8
 ENV MAKE="make -j\${NUMPROC}"
 ARG USE_MAKEFLAGS="-k V=0"

docker/Dockerfile

@@ -75,7 +75,7 @@ ARG MAKE_BUILD=make-build
 ################################################################################
 # Image containing the run-time dependencies for Sage                          #
 ################################################################################
-FROM ubuntu:jammy as run-time-dependencies
+FROM ubuntu:jammy AS run-time-dependencies
 LABEL maintainer="Erik M. Bray <[email protected]>, Julian Rüth <[email protected]>, Sebastian Oehms <[email protected]>"
 # Set sane defaults for common environment variables.
 ENV LC_ALL C.UTF-8
@@ -108,7 +108,7 @@ WORKDIR $HOME
 # Image containing everything so that a make in a clone of the Sage repository #
 # completes without errors                                                     #
 ################################################################################
-FROM run-time-dependencies as build-time-dependencies
+FROM run-time-dependencies AS build-time-dependencies
 # Install the build time dependencies & git & rdfind
 RUN sudo apt-get -qq update \
     && sudo apt-get -qq install -y wget build-essential automake m4 dpkg-dev python3 libssl-dev git rdfind \
@@ -118,7 +118,7 @@ RUN sudo apt-get -qq update \
 ################################################################################
 # Image with an empty git repository in $SAGE_ROOT.                            #
 ################################################################################
-FROM build-time-dependencies as source-clean
+FROM build-time-dependencies AS source-clean
 ARG SAGE_ROOT=/home/sage/sage
 RUN mkdir -p "$SAGE_ROOT"
 WORKDIR $SAGE_ROOT
@@ -135,7 +135,7 @@ RUN git remote add upstream https://github.com/sagemath/sage.git
 # build is not going to use the build-time-dependencies target but rely on     #
 # whatever tools are installed in ARTIFACT_BASE.                               #
 ################################################################################
-FROM $ARTIFACT_BASE as source-from-context
+FROM $ARTIFACT_BASE AS source-from-context
 WORKDIR $HOME
 COPY --chown=sage:sage . sage-context
 # Checkout the commit that is checked out in $HOME/sage-context
@@ -183,7 +183,7 @@ RUN patch -p1 < "$HOME"/sage-context.patch
 ################################################################################
 # Image with a built sage but without sage's documentation.                    #
 ################################################################################
-FROM source-from-context as make-build
+FROM source-from-context AS make-build
 # Make sure that the result runs on most CPUs.
 ENV SAGE_FAT_BINARY yes
 # Just to be sure Sage doesn't try to build its own GCC (even though
@@ -204,7 +204,7 @@ RUN make build
 ################################################################################
 # Image with a full build of sage and its documentation.                       #
 ################################################################################
-FROM $MAKE_BUILD as make-all
+FROM $MAKE_BUILD AS make-all
 # The docbuild needs quite some RAM (as of May 2018). It sometimes calls
 # os.fork() to spawn an external program which then exceeds easily the
 # overcommit limit of the system (no RAM is actually used, but this limit is
@@ -218,15 +218,15 @@ RUN make
 ################################################################################
 # Image with a full build of sage, ready to release.                           #
 ################################################################################
-FROM make-all as make-release
+FROM make-all AS make-release
 RUN make micro_release
 
 ################################################################################
 # A releasable (relatively small) image which contains a copy of sage without  #
 # temporary build artifacts which is set up to start the command line          #
 # interface if no parameters are passed in.                                    #
 ################################################################################
-FROM run-time-dependencies as sagemath
+FROM run-time-dependencies AS sagemath
 ARG HOME=/home/sage
 ARG SAGE_ROOT=/home/sage/sage
 COPY --chown=sage:sage --from=make-release $SAGE_ROOT/ $SAGE_ROOT/
@@ -244,7 +244,7 @@ CMD ["sage"]
 # Image with a full build of sage and its documentation but everything         #
 # stripped that can be quickly rebuild by make.                                #
 ################################################################################
-FROM make-all as make-fast-rebuild-clean
+FROM make-all AS make-fast-rebuild-clean
 # Of course, without site-packages/sage, sage does not start but copying/compiling
 # them from build/pkgs/sagelib/src/build is very fast.
 RUN make fast-rebuild-clean; \
@@ -255,7 +255,7 @@ RUN make fast-rebuild-clean; \
 # identical to make-fast-rebuild-clean or contains a "patch" which can be used #
 # to upgrade ARTIFACT_BASE to make-fast-rebuild-clean.                         #
 ################################################################################
-FROM make-fast-rebuild-clean as sagemath-dev-patch
+FROM make-fast-rebuild-clean AS sagemath-dev-patch
 ARG ARTIFACT_BASE=source-clean
 ARG SAGE_ROOT=/home/sage/sage
 # Build a patch containing of a tar file which contains all the modified files
@@ -277,12 +277,12 @@ RUN if [ x"$ARTIFACT_BASE" != x"source-clean" ]; then \
 # the build artifacts intact so developers can make changes and rebuild        #
 # quickly                                                                      #
 ################################################################################
-FROM $ARTIFACT_BASE as sagemath-dev
+FROM $ARTIFACT_BASE AS sagemath-dev
 ARG SAGE_ROOT=/home/sage/sage
 # If docker is backed by aufs, then the following command adds the size of
 # ARTIFACT_BASE to the image size. As of mid 2018 this is notably the case with
 # the docker instances provided by setup_remote_docker on CircleCI. As a
-# result, the sagemath-dev images that are "build-from-latest" are twice as big
+# result, the sagemath-dev images that are "build-from-latest" are twice AS big
 # as the ones that are build on GitLab:
 # https://github.com/moby/moby/issues/6119#issuecomment-268870519
 COPY --chown=sage:sage --from=sagemath-dev-patch $SAGE_ROOT $SAGE_ROOT