fix: Ensure pip is available in venv for runtime installation (#12667)

## Summary

Fixes #12651

The Docker container was failing at startup with:
```
/ragflow/.venv/bin/python3: No module named pip
```

This occurred when `USE_DOCLING=true` because the `entrypoint.sh` tries
to use `uv pip install` to install docling at runtime.

## Root Cause

As explained in the issue:
1. `uv sync` creates a minimal, production-focused environment **without
pip**
2. The production stage copies the venv from builder
3. Runtime commands using `uv pip install` fail because pip is not
present

## Solution

Added `python -m ensurepip --upgrade` after `uv sync` in the Dockerfile
to ensure pip is available in the virtual environment:

```dockerfile
uv sync --python 3.12 --frozen && \
# Ensure pip is available in the venv for runtime package installation (fixes #12651)
.venv/bin/python3 -m ensurepip --upgrade
```

This is a minimal change that:
- Ensures pip is installed during build time
- Doesn't change any other behavior
- Allows runtime package installation via `uv pip install` to work

---
This is a Gittensor contribution.
gittensor:user:GlobalStar117

Co-authored-by: GlobalStar117 <GlobalStar117@users.noreply.github.com>

Dockerfile

@@ -154,7 +154,9 @@ RUN --mount=type=cache,id=ragflow_uv,target=/root/.cache/uv,sharing=locked \
     else \
         sed -i 's|pypi.tuna.tsinghua.edu.cn|pypi.org|g' uv.lock; \
     fi; \
-    uv sync --python 3.12 --frozen
+    uv sync --python 3.12 --frozen && \
+    # Ensure pip is available in the venv for runtime package installation (fixes #12651)
+    .venv/bin/python3 -m ensurepip --upgrade
 
 COPY web web
 COPY docs docs