fix(docker): install docker CLI for box backend; bump to 4.10.0-beta.3

The langbot_box service drives sandbox containers through the docker CLI
(CLISandboxBackend shells out to `docker run`/`docker exec`), but the
image shipped without a docker client, so DockerBackend.is_available()
was always false and the Box sandbox backend was unavailable in Docker
deployments — disabling native tools, skill execution and stdio MCP.
Install docker-ce-cli (client only) in the image, arch-aware so
multi-arch builds work.

Also bump langbot-plugin pin to 0.4.1, which disables proxy
auto-detection on internal control-plane WebSocket connections (the
langbot<->plugin_runtime / langbot<->box handshakes were failing on
hosts that inject a proxy into containers).

Bumps version to 4.10.0-beta.3.

src/langbot/__init__.py

@@ -1,3 +1,3 @@
 """LangBot - Production-grade platform for building agentic IM bots"""
 
-__version__ = '4.10.0-beta.2'
+__version__ = '4.10.0-beta.3'