[CI/Build] Allow mounting AWS credentials for sccache S3 auth (#35912)

Signed-off-by: Amr Mahdi <amrmahdi@meta.com>

docker/Dockerfile

@@ -262,7 +262,9 @@ RUN --mount=type=cache,target=/root/.cache/uv \
 
 # Build the vLLM wheel
 # if USE_SCCACHE is set, use sccache to speed up compilation
+# AWS credentials mounted at ~/.aws/credentials for sccache S3 auth (optional)
 RUN --mount=type=cache,target=/root/.cache/uv \
+    --mount=type=secret,id=aws-credentials,target=/root/.aws/credentials,required=false \
     if [ "$USE_SCCACHE" = "1" ]; then \
         echo "Installing sccache..." \
         && case "${TARGETPLATFORM}" in \