refactor: replace runtime.dockerfile with AgentProvider.dockerfile property

Drop the `dockerfile` field from `AgentProviderRuntime` and replace it
with a convention-based `dockerfile` property on `AgentProvider`: the
base class looks for a `Dockerfile` file next to the provider's own
`agent_provider.py` module (via `inspect.getfile`), returning its path
or None. Built-in providers inherit the default automatically; custom
user providers work the same way by dropping a Dockerfile next to their
plugin file; any provider needing a non-standard path can override.

All callers (`docker/prepare.py`, `smolmachines/prepare.py`,
`capability_apply.py`) now resolve the provider object once and call
`.dockerfile` directly instead of reading `runtime.dockerfile`.

bot_bottle/backend/docker/capability_apply.py

@@ -34,6 +34,7 @@ import shutil
 import subprocess
 from pathlib import Path
 
+from ...agent_provider import get_provider
 from ...log import info, warn
 from .bottle_state import (
     mark_preserved,
@@ -93,11 +94,11 @@ def fetch_current_dockerfile(slug: str) -> str:
     override = per_bottle_dockerfile(slug)
     if override is not None:
         return override
-    repo_dockerfile = _repo_dockerfile_path()
-    if repo_dockerfile.is_file():
+    repo_dockerfile = get_provider("claude").dockerfile
+    if repo_dockerfile is not None and repo_dockerfile.is_file():
         return repo_dockerfile.read_text()
     raise CapabilityApplyError(
-        f"no per-bottle Dockerfile for {slug} and no repo Dockerfile at "
+        f"no per-bottle Dockerfile for {slug} and no provider Dockerfile at "
         f"{repo_dockerfile}"
     )
 
@@ -125,12 +126,6 @@ def apply_capability_change(slug: str, new_dockerfile: str) -> tuple[str, str]:
 # --- Internals -------------------------------------------------------------
 
 
-def _repo_dockerfile_path() -> Path:
-    """Path to the Claude provider Dockerfile. Resolved at call time so
-    the path is correct regardless of where this module is imported from."""
-    # bot_bottle/backend/docker/ -> bot_bottle/ -> contrib/claude/Dockerfile
-    return Path(__file__).resolve().parent.parent.parent / "contrib" / "claude" / "Dockerfile"
-
 
 def snapshot_transcript(slug: str) -> None:
     """`docker cp` /home/node/.claude out of the agent container into

bot_bottle/backend/docker/prepare.py

@@ -15,7 +15,7 @@ from datetime import datetime, timezone
 from dataclasses import replace
 from pathlib import Path
 
-from ...agent_provider import PROVIDER_TEMPLATES, agent_provision_plan, runtime_for
+from ...agent_provider import PROVIDER_TEMPLATES, agent_provision_plan, get_provider
 from ...egress import Egress
 from ...env import ResolvedEnv, resolve_env
 from ...git_gate import GitGate
@@ -59,7 +59,8 @@ def resolve_plan(
     agent = manifest.agents[spec.agent_name]
     bottle = manifest.bottle_for(spec.agent_name)
     provider = bottle.agent_provider
-    provider_runtime = runtime_for(provider.template)
+    provider_obj = get_provider(provider.template)
+    provider_runtime = provider_obj.runtime
     guest_home = "/home/node"
     workspace_plan = resolve_workspace_plan(spec, guest_home=guest_home)
 
@@ -99,20 +100,16 @@ def resolve_plan(
     elif provider.dockerfile:
         image_default = f"bot-bottle-{provider.template}:{slug}"
         dockerfile_path = _resolve_manifest_dockerfile(provider.dockerfile, spec)
-    elif provider_runtime.dockerfile:
-        image_default = provider_runtime.image
-        dockerfile_path = provider_runtime.dockerfile
-    elif provider.template not in PROVIDER_TEMPLATES:
-        user_dockerfile = (
-            Path.home() / ".bot-bottle" / "contrib" / provider.template / "Dockerfile"
-        )
-        if user_dockerfile.is_file():
-            image_default = f"bot-bottle-{provider.template}:{slug}"
-            dockerfile_path = str(user_dockerfile)
+    else:
+        p_dockerfile = provider_obj.dockerfile
+        if p_dockerfile is not None:
+            if provider.template in PROVIDER_TEMPLATES:
+                image_default = provider_runtime.image
+            else:
+                image_default = f"bot-bottle-{provider.template}:{slug}"
+            dockerfile_path = str(p_dockerfile)
         else:
             image_default = provider_runtime.image
-    else:
-        image_default = provider_runtime.image
     image = os.environ.get("BOT_BOTTLE_IMAGE", image_default)
     derived_image = ""
     runtime_image = image
@@ -216,13 +213,11 @@ def resolve_plan(
         # moved it behind the `list-egress-routes` MCP tool so the
         # agent gets live state rather than a launch-time snapshot.)
         supervise_dockerfile_path = (
-            Path(dockerfile_path)
-            if dockerfile_path
-            else Path(__file__).resolve().parent.parent.parent / "contrib" / "claude" / "Dockerfile"
+            Path(dockerfile_path) if dockerfile_path else provider_obj.dockerfile
         )
         dockerfile_content = (
             supervise_dockerfile_path.read_text(encoding="utf-8")
-            if supervise_dockerfile_path.is_file()
+            if supervise_dockerfile_path is not None and supervise_dockerfile_path.is_file()
             else ""
         )
         supervise_dir = supervise_state_dir(slug)