Always use cache mounts when installing vllm to avoid populating pip cache in...

Always use cache mounts when installing vllm to avoid populating pip cache in the image. Also remove apt cache. (#23270)
Signed-off-by: Valentyn Tymofieiev <valentyn@google.com>

docker/Dockerfile.tpu

@@ -7,7 +7,8 @@ WORKDIR /workspace/vllm
 # Install some basic utilities
 RUN apt-get update && apt-get install -y \
     git \
-    ffmpeg libsm6 libxext6 libgl1
+    ffmpeg libsm6 libxext6 libgl1 && \
+    rm -rf /var/lib/apt/lists/*
 
 # Build vLLM.
 COPY . .
@@ -16,6 +17,9 @@ RUN --mount=type=bind,source=.git,target=.git \
     if [ "$GIT_REPO_CHECK" != 0 ]; then bash tools/check_repo.sh; fi
 
 # Remove existing versions of dependencies
+# TODO: These packages will remain as dead weight in the Docker image layers.
+# We should find a way to build the image without uninstalling these.
+# Consider using a different base image.
 RUN pip uninstall -y torch torch_xla torchvision
 
 ENV VLLM_TARGET_DEVICE="tpu"
@@ -23,9 +27,10 @@ RUN --mount=type=cache,target=/root/.cache/pip \
     --mount=type=bind,source=.git,target=.git \
     python3 -m pip install \
         -r requirements/tpu.txt
-RUN python3 -m pip install -e .
+
+RUN --mount=type=cache,target=/root/.cache/pip python3 -m pip install -e .
 
 # install development dependencies (for testing)
-RUN python3 -m pip install -e tests/vllm_test_utils
+RUN --mount=type=cache,target=/root/.cache/pip python3 -m pip install -e tests/vllm_test_utils
 
 CMD ["/bin/bash"]