Use '.python-version' for builds

Dockerfile

@@ -1,4 +1,6 @@
-FROM ghcr.io/astral-sh/uv:python3.13-bookworm-slim AS builder
+ARG PYTHON_VERSION="3.13"
+
+FROM ghcr.io/astral-sh/uv:python${PYTHON_VERSION}-bookworm-slim AS builder
 ENV UV_COMPILE_BYTECODE=1 UV_LINK_MODE=copy
 ENV UV_PYTHON_DOWNLOADS=0
 WORKDIR /app
@@ -11,7 +13,7 @@ RUN --mount=type=cache,target=/root/.cache/uv \
     uv sync --locked --no-dev
 
 
-FROM python:3.13-slim-bookworm
+FROM python:${PYTHON_VERSION}-slim-bookworm
 COPY --from=builder --chown=app:app /app /app
 ENV PATH="/app/.venv/bin:$PATH"
 EXPOSE 80/tcp