refactor(docker): make pipelock proxy a per-instance attribute

Avoids cross-instance state via class attribute; the proxy is now
constructed in __init__ alongside its owning backend.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/backend.py

@@ -59,7 +59,9 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
     (default)."""
 
     name = "docker"
-    _proxy: DockerPipelockProxy = DockerPipelockProxy()
+
+    def __init__(self) -> None:
+        self._proxy = DockerPipelockProxy()
 
     def prepare(self, spec: BottleSpec, *, stage_dir: Path) -> DockerBottlePlan:
         """Resolve names, validate, write scratch files. No Docker