fix(orchestrator): gateway image builds cache-aware, not build-if-missing

The cut-over dropped compose (whose `build:` rebuilt the sidecar image on
every up), and DockerGateway.ensure_built was build-if-missing — so a change
to the gateway's flat sources (egress addon / git-http / policy_resolver /
supervise) left a STALE image silently running the old single-tenant daemons
(this bit e2e validation). ensure_built now always `docker build`s
(cache-aware: a cache check when nothing changed, a real rebuild when sources
moved), matching the old compose behavior. BOT_BOTTLE_NO_CACHE forces a full
rebuild (parity with #354's `start --no-cache`).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/orchestrator/gateway.py

@@ -109,17 +109,23 @@ class DockerGateway(Gateway):
         return run_docker(["docker", "image", "inspect", self.image_ref]).returncode == 0
 
     def ensure_built(self) -> None:
-        """Build the bundle image from its Dockerfile when it's missing.
-        No-op when the image is already present, or when no dockerfile is
-        configured (e.g. a pre-pulled image)."""
-        if self._dockerfile is None or self.image_exists():
+        """Build the bundle image from its Dockerfile, **cache-aware** — cheap
+        (a cache check) when nothing changed, a real rebuild when the flat
+        sources (egress addon / git-http / policy_resolver / supervise) moved.
+
+        This deliberately builds every time rather than build-if-missing: the
+        per-bottle model kept the image fresh via compose's `build:` on up, and
+        a stale image silently runs the OLD single-tenant daemons. No-op only
+        when no dockerfile is configured (a pre-pulled image). BOT_BOTTLE_NO_CACHE
+        forces a full rebuild (parity with `start --no-cache`)."""
+        if self._dockerfile is None:
             return
-        proc = run_docker([
-            "docker", "build",
-            "-t", self.image_ref,
-            "-f", str(self._build_context / self._dockerfile),
-            str(self._build_context),
-        ])
+        argv = ["docker", "build", "-t", self.image_ref,
+                "-f", str(self._build_context / self._dockerfile),
+                str(self._build_context)]
+        if os.environ.get("BOT_BOTTLE_NO_CACHE"):
+            argv.insert(2, "--no-cache")
+        proc = run_docker(argv)
         if proc.returncode != 0:
             raise GatewayError(f"gateway image build failed: {proc.stderr.strip()}")