Recover from preview failures faster

Port and bind problems

• Listening only on localhost stops Fly edge routing—bind to 0.0.0.0.
• internal_port must match Dockerfile EXPOSE and the workload's listeners.
• Previewly forcibly merges PORT—ignore stale hard-coded literals.

Configuration mistakes

• Malformed YAML/TOML produces parse errors surfaced in Deployment build logs.
• If automatic detection guesses the wrong ecosystem, declare runtime plus startup_command explicitly—or switch entirely to authoring a Dockerfile reference.
• Python services almost always need an explicit starter because requirements vary widely.

Timeouts and bulky contexts

• Large Git contexts slow remote Docker builds—add .dockerignore aggressively.
• Consider breaking huge monorepos into slimmer layers or shipping prebuilt artifacts when permitted.

Verify locally

When debugging an authored Dockerfile, run docker build and docker run -p with the same environment variables you expect Fly to ship. For generated builds, capture Previewly's emitted Dockerfile from build logs and reproduce locally.