BottleBackend.prepare was calling resolve_manifest_dockerfile("", spec)
for every bottle where the manifest did not set agent_provider.dockerfile.
That resolves an empty string against user_cwd, returning the cwd
itself — which docker then tried to read as a Dockerfile, giving
"is a directory" errors during image build.
When the manifest doesn't override, use the provider plugin's bundled
Dockerfile path (next to its agent_provider.py module) — mirroring
the pre-refactor behavior.
didericis-claude
e2514b3885