[GitHub][docker] Fix 'FromAsCasing' violation in CI dockerfile (llvm#161017)

Fixes https://docs.docker.com/reference/build-checks/from-as-casing/
which also gives warning when building container from command line:
```
=> WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 1)                                                                                        
=> WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 4)                                                                                        
=> WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 40)                                                                                       
=> WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 100)
```

Author: vbvictor

.github/workflows/containers/github-action-ci/Dockerfile

@@ -1,7 +1,7 @@
-FROM docker.io/library/ubuntu:24.04 as base
+FROM docker.io/library/ubuntu:24.04 AS base
 ENV LLVM_SYSROOT=/opt/llvm
 
-FROM base as stage1-toolchain
+FROM base AS stage1-toolchain
 ENV LLVM_VERSION=21.1.1
 
 RUN apt-get update && \
@@ -37,7 +37,7 @@ RUN cmake -B ./build -G Ninja ./llvm \
 
 RUN ninja -C ./build stage2-clang-bolt stage2-install-distribution && ninja -C ./build install-distribution
 
-FROM base as ci-container
+FROM base AS ci-container
 
 COPY --from=stage1-toolchain $LLVM_SYSROOT $LLVM_SYSROOT
 
@@ -97,7 +97,7 @@ RUN echo '%sudo ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
 USER gha
 WORKDIR /home/gha
 
-FROM ci-container as ci-container-agent
+FROM ci-container AS ci-container-agent
 
 ENV GITHUB_RUNNER_VERSION=2.328.0