feat(smolmachines): build agent image from repo Dockerfile (PRD 0023 chunk 4c)

Replaces the alpine:latest placeholder with a real claude-bottle
agent image, converted into a .smolmachine artifact via an
ephemeral local OCI registry.

Why the registry hop: smolvm pack create only accepts OCI registry
refs. Empirically it rejects docker-daemon://, oci-layout://,
docker-archive: tarballs, and every other transport tested — the
crane backend treats anything with a scheme prefix as a registry
hostname. To convert a locally-built docker image into a
.smolmachine we have to push it somewhere smolvm can pull from.
Smallest path: bring up registry:2.8.3 bound to 127.0.0.1:<random>,
docker tag + docker push into it, smolvm pack create --image
localhost:<port>/claude-bottle:<id>, tear down the registry.

The .smolmachine is cached under
~/.cache/claude-bottle/smolmachines/ keyed by the docker image ID
(first 16 hex chars of the sha256), so a Dockerfile change picks
up a new image ID and invalidates the cache. Unchanged rebuilds
skip the whole build → registry → pack pipeline.

This puts `docker build` in smolmachines prepare (the docker
backend defers it to launch). Necessary because pack_create needs
the image ID to derive the cache key, and prepare is the only
hook ahead of launch that runs once per slug.

Adds:
- claude_bottle/backend/docker/util.py: image_id / tag / push
  helpers (thin docker CLI wrappers).
- claude_bottle/backend/smolmachines/local_registry.py:
  ephemeral_registry() context manager; pins registry:2.8.3 by
  digest, binds 127.0.0.1::5000 (loopback-only), force-removes on
  exit.

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

claude_bottle/backend/docker/util.py

@@ -147,6 +147,39 @@ def build_image_with_cwd(derived: str, base: str, cwd: str) -> None:
     )
 
 
+def image_id(ref: str) -> str:
+    """Return the content-addressed image ID (e.g.
+    `sha256:abcd...`) for `ref`. The smolmachines backend keys its
+    `.smolmachine` artifact cache on this, so a Dockerfile change
+    that produces a new image automatically invalidates the cache."""
+    r = subprocess.run(
+        ["docker", "image", "inspect", "--format", "{{.Id}}", ref],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if r.returncode != 0:
+        die(
+            f"docker image inspect for {ref!r} failed: "
+            f"{(r.stderr or '').strip() or '<no stderr>'}"
+        )
+    return r.stdout.strip()
+
+
+def tag(src: str, dst: str) -> None:
+    """`docker tag SRC DST`. Idempotent. Used by smolmachines prepare
+    to retag the locally-built image into a localhost:<port>/... ref
+    that the ephemeral registry will accept."""
+    subprocess.run(["docker", "tag", src, dst], check=True)
+
+
+def push(ref: str) -> None:
+    """`docker push REF`. Used by smolmachines prepare to push the
+    agent image into the ephemeral local registry so smolvm's crane
+    backend can pull it."""
+    subprocess.run(["docker", "push", ref], check=True)
+
+
 def _silent_run(cmd: Iterable[str]) -> int:
     return subprocess.run(
         list(cmd),