The manifest-based flow was building the Docker image inside start_instance,
which made the start HTTP request take 3-5 minutes (downloading ubuntu:24.04,
apt-get update, installing packages, Node.js, npm packages). The frontend
showed a spinner forever because the HTTP request was still pending.
Move the image build to create_instance (same pattern as dockerfile types):
1. create_instance now compiles Dockerfile + entrypoint and builds the image
2. start_instance sees the image already exists and skips the build
3. Start is fast — just docker compose up + health checks
This matches the UX expectation: creation has a spinner (can be slow),
start should be quick.
Alex Blank
f4802ece4d