Fix FromAsCasing warnings in Dockerfile

Warning was:
  FromAsCasing: 'as' and 'FROM' keywords' casing do not match
  More info: https://docs.docker.com/go/dockerfile/rule/from-as-casing/

docker/dev/backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:3.3.4-bullseye as develop
+FROM ruby:3.3.4-bullseye AS develop
 MAINTAINER [email protected]
 
 ARG DEV_UID=1000
@@ -67,7 +67,7 @@ RUN gem install bundler --version "${BUNDLER_VERSION}" --no-document
 
 ####### Testing image below #########
 
-FROM develop as test
+FROM develop AS test
 
 USER root
 

docker/prod/Dockerfile

@@ -1,5 +1,5 @@
 ARG RUBY_VERSION="3.3.4"
-FROM ruby:${RUBY_VERSION}-bookworm as base
+FROM ruby:${RUBY_VERSION}-bookworm AS base
 LABEL maintainer="[email protected]"
 
 ARG BUNDLER_VERSION="2.5.13"
@@ -80,7 +80,7 @@ RUN cp Gemfile.lock.bak Gemfile.lock && rm Gemfile.lock.bak && \
 # -------------------------------------
 # slim (public)
 # -------------------------------------
-FROM base as slim
+FROM base AS slim
 
 USER $APP_USER
 EXPOSE 8080
@@ -91,7 +91,7 @@ VOLUME ["$APP_DATA_PATH"]
 # -------------------------------------
 # all-in-one (public)
 # -------------------------------------
-FROM base as all-in-one
+FROM base AS all-in-one
 
 ENV OPENPROJECT_RAILS__CACHE__STORE=memcache
 ENV DATABASE_URL=postgres://openproject:[email protected]/openproject