[Build] Defer flashinfer cubin download to avoid ~2.5 GB (decompressed) layer duplication (#41134)

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

docker/Dockerfile

@@ -585,9 +585,7 @@ RUN --mount=type=cache,target=/root/.cache/uv \
 ARG FLASHINFER_VERSION=0.6.8.post1
 RUN --mount=type=cache,target=/root/.cache/uv \
     uv pip install --system flashinfer-jit-cache==${FLASHINFER_VERSION} \
-        --extra-index-url https://flashinfer.ai/whl/cu$(echo $CUDA_VERSION | cut -d. -f1,2 | tr -d '.') \
-    && flashinfer show-config \
-    && flashinfer download-cubin
+        --extra-index-url https://flashinfer.ai/whl/cu$(echo $CUDA_VERSION | cut -d. -f1,2 | tr -d '.')
 
 # ============================================================
 # OPENAI API SERVER DEPENDENCIES
@@ -669,6 +667,13 @@ RUN --mount=type=bind,from=build,src=/tmp/ep_kernels_workspace/dist,target=/vllm
     uv pip install --system ep_kernels/dist/*.whl --verbose \
         --extra-index-url ${PYTORCH_CUDA_INDEX_BASE_URL}/cu$(echo $CUDA_VERSION | cut -d. -f1,2 | tr -d '.')
 
+# Download FlashInfer precompiled cubins AFTER all pip installs are done.
+# This must run after the vLLM wheel and EP kernels installs above, because
+# those can reinstall/touch flashinfer packages. Downloading cubins earlier
+# (in the flashinfer-jit-cache layer) causes ~2.5 GB of layer duplication
+# when a later pip install overwrites flashinfer package files.
+RUN flashinfer show-config && flashinfer download-cubin
+
 # CUDA image changed from /usr/local/nvidia to /usr/local/cuda in 12.8 but will
 # return to /usr/local/nvidia in 13.0 to allow container providers to mount drivers
 # consistently from the host (see https://github.com/vllm-project/vllm/issues/18859).