[docker] Fix downloading sccache on aarch64 platform (#30070)

Signed-off-by: Nick Cao <nickcao@nichi.co>

docker/Dockerfile

@@ -183,7 +183,7 @@ ARG nvcc_threads=8
 ENV NVCC_THREADS=$nvcc_threads
 
 ARG USE_SCCACHE
-ARG SCCACHE_DOWNLOAD_URL=https://github.com/mozilla/sccache/releases/download/v0.8.1/sccache-v0.8.1-x86_64-unknown-linux-musl.tar.gz
+ARG SCCACHE_DOWNLOAD_URL
 ARG SCCACHE_ENDPOINT
 ARG SCCACHE_BUCKET_NAME=vllm-build-sccache
 ARG SCCACHE_REGION_NAME=us-west-2
@@ -201,10 +201,16 @@ ENV SETUPTOOLS_SCM_PRETEND_VERSION="0.0.0+csrc.build"
 RUN --mount=type=cache,target=/root/.cache/uv \
     if [ "$USE_SCCACHE" = "1" ]; then \
         echo "Installing sccache..." \
+        && case "${TARGETPLATFORM}" in \
+          linux/arm64) SCCACHE_ARCH="aarch64" ;; \
+          linux/amd64) SCCACHE_ARCH="x86_64" ;; \
+          *) echo "Unsupported TARGETPLATFORM for sccache: ${TARGETPLATFORM}" >&2; exit 1 ;; \
+        esac \
+        && export SCCACHE_DOWNLOAD_URL="${SCCACHE_DOWNLOAD_URL:-https://github.com/mozilla/sccache/releases/download/v0.8.1/sccache-v0.8.1-${SCCACHE_ARCH}-unknown-linux-musl.tar.gz}" \
         && curl -L -o sccache.tar.gz ${SCCACHE_DOWNLOAD_URL} \
         && tar -xzf sccache.tar.gz \
-        && sudo mv sccache-v0.8.1-x86_64-unknown-linux-musl/sccache /usr/bin/sccache \
-        && rm -rf sccache.tar.gz sccache-v0.8.1-x86_64-unknown-linux-musl \
+        && sudo mv sccache-v0.8.1-${SCCACHE_ARCH}-unknown-linux-musl/sccache /usr/bin/sccache \
+        && rm -rf sccache.tar.gz sccache-v0.8.1-${SCCACHE_ARCH}-unknown-linux-musl \
         && if [ ! -z ${SCCACHE_ENDPOINT} ] ; then export SCCACHE_ENDPOINT=${SCCACHE_ENDPOINT} ; fi \
         && export SCCACHE_BUCKET=${SCCACHE_BUCKET_NAME} \
         && export SCCACHE_REGION=${SCCACHE_REGION_NAME} \