Fix FROM as casing warning (#2449)

ci/Dockerfile.debian

@@ -1,7 +1,7 @@
 # Base Target to build and install all needed base configuration and packages. Specifie the needed platform with the docker '--platform XXX' option
 ARG DEBIAN_CODENAME=bullseye
 ARG BASE_TEST_IMAGE=test-code
-FROM debian:${DEBIAN_CODENAME}-slim as base
+FROM debian:${DEBIAN_CODENAME}-slim AS base
 ARG DEBIAN_CODENAME
 
 ENV DOCKER_RUNNING=true
@@ -34,7 +34,7 @@ RUN export DEBIAN_FRONTEND=noninteractive \
 # ------
 
 # Base Target for setting up the default user. user can be selected with the docker '--user YYY' option
-FROM base as user
+FROM base AS user
 ARG USER=pi
 ARG USER_GROUP=$USER
 
@@ -50,7 +50,7 @@ RUN groupadd --gid 1000 $USER_GROUP \
 ####### Code Targets #######
 
 # Target for adding code from the repo. Set Default User
-FROM user as code
+FROM user AS code
 
 COPY --chown=$USER:$USER_GROUP --chmod=770 . /code
 WORKDIR /code
@@ -63,7 +63,7 @@ USER $USER
 ####### Test Targets #######
 
 # Target for setting up an alternativ user 'hans:wurst'. user can be selected with the docker '--user YYY' option
-FROM user as test-user
+FROM user AS test-user
 
 ENV TEST_USER_GROUP=test
 
@@ -79,7 +79,7 @@ RUN export USER_ALT=hans \
 
 
 # Target for adding envs and scripts from the repo to test installation
-FROM test-user as test-code
+FROM test-user AS test-code
 ARG GIT_BRANCH
 ARG GIT_URL
 
@@ -104,7 +104,7 @@ COPY --chown=root:$TEST_USER_GROUP --chmod=770 scripts/installscripts/tests/*.sh
 
 
 # Target for applying latest updates (should not be cached!)
-FROM $BASE_TEST_IMAGE as test-update
+FROM $BASE_TEST_IMAGE AS test-update
 RUN export DEBIAN_FRONTEND=noninteractive \
   && apt-get update \
   && apt-get -y upgrade \