build(docker): appuser-owned data directories; note uv's index strategy for the docling file

A named volume mounted on /app/inputs, /app/indexes or /app/vectors inherits
the ownership of the image directory, so the image creates them as appuser;
before this the volume came up root-owned and every upload failed with a
permission error unless the container ran as root. docker-compose-standalone.yaml
still runs backend and worker as root, like docker-compose-hub.yaml, so it
also works with image tags that predate these directories.

requirements-docling.txt adds the PyTorch CPU index, which uv resolves only
with UV_INDEX_STRATEGY=unsafe-best-match (pip is unaffected); the file header
and the docs say so and point uv users at uv sync --extra docling.

docs/content/Deploying/Development-Environment.mdx

@@ -108,6 +108,10 @@ To run the DocsGPT backend locally, you'll need to set up a Python environment a
     # or with uv: uv sync --extra docling --extra milvus
     ```
 
+    The docling file adds the PyTorch CPU index; pip handles that as-is, while
+    `uv pip install -r` needs `UV_INDEX_STRATEGY=unsafe-best-match` (or use
+    `uv sync --extra docling`, which reads the lock).
+
     A feature whose extra is missing fails with the exact command to run.
     After changing `pyproject.toml`, run `uv lock` and
     `bash scripts/export_requirements.sh` so the exported files stay in sync