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.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "langbot"
-version = "4.10.0-beta.2"
+version = "4.10.0-beta.3"
 description = "Production-grade platform for building agentic IM bots"
 readme = "README.md"
 license-files = ["LICENSE"]
@@ -70,7 +70,7 @@ dependencies = [
     "chromadb>=1.0.0,<2.0.0",
     "qdrant-client (>=1.15.1,<2.0.0)",
     "pyseekdb==1.1.0.post3",
-    "langbot-plugin==0.4.0",
+    "langbot-plugin==0.4.1",
     "asyncpg>=0.30.0",
     "line-bot-sdk>=3.19.0",
     "matrix-nio>=0.25.2",