build: multi-stage Docker image with uv and BuildKit caching (#1261)

* build: multi-stage Docker image with uv and BuildKit caching

Builder/runtime split keeps the toolchain out of the runtime image
(standalone ~1.8GB -> ~1.2GB). Dependencies install with uv from
uv.lock in a cache-mounted, source-independent layer. Runtime keeps
the editable install because akkudoktoreos.core.version needs the
src/ layout. Also fixes the io.hass.version label and adds a
HEALTHCHECK.

* build: address Docker image review feedback

- Healthcheck honours EOS_SERVER__PORT instead of hardcoding 8503.
- BUILD_VERSION defaults to "dev" rather than the literal "VERSION";
  docker-compose passes the real version, and the redundant
  org.opencontainers.image.version label is dropped (CI sets it via
  docker/metadata-action).

* fix: use resolved server port for container healthcheck

---------

Co-authored-by: Normann <github@koldrack.com>
Co-authored-by: Normann Koldrack <normann.koldrack@desy.de>

.github/workflows/docker-build.yml

@@ -119,6 +119,8 @@ jobs:
         uses: docker/build-push-action@v7
         with:
           context: .
+          build-args: |
+            BUILD_VERSION=${{ steps.meta.outputs.version }}
           platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
           annotations: ${{ steps.meta.outputs.annotations }}