Set MPLCONFIGDIR for matplotlib in nobody container

The nobody user has no writable home directory, so matplotlib falls
back to creating a temporary cache dir on every import. Setting
MPLCONFIGDIR to /tmp/matplotlib gives it a stable writable path.

Dockerfile.mcp

@@ -14,6 +14,7 @@ WORKDIR /app
 
 ENV UV_COMPILE_BYTECODE=1
 ENV PATH="/app/.venv/bin:$PATH"
+ENV MPLCONFIGDIR=/tmp/matplotlib
 
 COPY --from=deps /app/.venv /app/.venv
 COPY src/ src/