Fix Dockerfile: use named stage for uv version pinning

COPY --from does not support ARG expansion, so use a separate FROM
stage to reference the pinned uv image.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sandhose

docker/Dockerfile

@@ -10,12 +10,14 @@ ARG DEBIAN_VERSION=bookworm
 ARG PYTHON_VERSION=3.12
 ARG UV_VERSION=0.11.3
 
+FROM ghcr.io/astral-sh/uv:${UV_VERSION} AS uv
+
 ###
 ### Stage 0: builder
 ###
 FROM python:${PYTHON_VERSION}-slim-${DEBIAN_VERSION} AS builder
 
-COPY --from=ghcr.io/astral-sh/uv:${UV_VERSION} /uv /uvx /bin/
+COPY --from=uv /uv /uvx /bin/
 
 WORKDIR /app