Previewly documentation

Spin up ephemeral preview environments from pull requests with minimal setup. Previewly clones your repo, builds a container on Fly Machines, assigns a HTTPS URL, and tears the environment down when you are finished.

Topics

Getting started — configure the GitHub App, defaults for new repos, and the basic preview lifecycle.
Configuration ( previewly.toml / previewly.yml / yaml) — declare port, startup command, environment variables, and optional Dockerfile path without editing platform settings.
Custom Dockerfile (optional) — when you want full image control beyond what auto-detection provides.
Troubleshooting — preview failed to bind, timeouts, config syntax, and hygiene tips.

You do not need a Dockerfile in your repository anymore: Previewly can infer common stacks from files like package.json or go.mod and wire a sensible default image. Provide a Dockerfile only when you need a bespoke build.