Question 1

What does it mean to "run a Docker app on Fly.io"?

Accepted Answer

On Fly.io, your Dockerfile is built into an OCI image and then run inside a Fly Machine, which is a hardware-virtualized microVM. From your code's perspective it's the same Linux environment your Dockerfile describes, but the underlying isolation is closer to a tiny VM than a shared-kernel container. The upshot is that any standard Dockerfile works without changes.

Question 2

How do I deploy a Dockerized app to Fly.io?

Accepted Answer

Install flyctl, then run fly launch from the directory containing your Dockerfile. The launcher detects it, builds it (remotely by default), provisions a Fly app, and generates a fly.toml. Subsequent updates ship with fly deploy. If you want to inspect the generated config before the first deploy, run fly launch --no-deploy and edit fly.toml first.

Question 3

Can I deploy a pre-built Docker image instead of building from source?

Accepted Answer

Yes. Tag your image for the Fly.io registry (docker tag myapp:latest registry.fly.io/:latest), run fly auth docker once to log Docker into the registry, push the image, and then deploy with fly deploy --image registry.fly.io/:latest. You can also pull from a public registry like Docker Hub by passing the public reference to --image.

Question 4

How does Fly.io's private Docker registry work?

Accepted Answer

Each Fly.io app gets a path at registry.fly.io/<app-name>. Access is scoped to your organization, so any app in the same org can pull from any other app's registry path. Authentication happens via fly auth docker, which writes a short-lived token to ~/.docker/config.json. The token expires after a few minutes, so it's safe to leave the command in a CI script.

Question 5

Why is my Dockerfile build failing with exit code 1 on Fly.io?

Accepted Answer

Exit code 1 means the Dockerfile itself failed, not the deploy layer. Reproduce it locally with docker build . to get the same error in your own terminal. Once it builds cleanly there, it'll build cleanly on Fly.io. For more detail during the deploy, run LOG_LEVEL=debug fly deploy.

Question 6

Can I build my Docker image locally and push it instead of using Fly.io's remote builder?

Accepted Answer

Yes. Run fly deploy --local-only to build the image on your machine and push it to the Fly.io registry from there. This is also the fallback when the remote builder is slow or unavailable. The flip side is that you need Docker running locally and a fast enough upload, since the whole image goes over your connection instead of being built on Fly infrastructure.

Question 7

Does my Dockerfile need a CMD or ENTRYPOINT?

Accepted Answer

Yes. Fly Machines start the image's CMD or ENTRYPOINT exactly the way docker run would. If the container exits immediately when you test it with docker run <your-image> locally, it'll exit immediately on Fly.io too. Make sure the process stays in the foreground and binds to 0.0.0.0 on the port declared in fly.toml.

Question 8

Can I run Docker-in-Docker on Fly.io?

Accepted Answer

Yes. Fly Machines are hardware-virtualized, so the Docker daemon runs cleanly inside one without the privilege escalations that container-in-container setups normally require. This is what makes Fly a reasonable host for sandboxes, CI runners, and AI agents that need to launch their own containers.

Question 9

Can I share a Docker image between multiple apps in my Fly.io organization?

Accepted Answer

Yes. Registry paths look per-app (registry.fly.io/), but access is checked at the organization level. You can build and push once to registry.fly.io/ and then reference the same image from a sibling app via fly deploy --image registry.fly.io/:. Tag conventions are up to you.

Question 10

What if my project doesn't have a Dockerfile?

Accepted Answer

fly launch detects common frameworks (Rails, Phoenix, Django, Next.js, SvelteKit, and others) and generates a working Dockerfile for them. If your project is something fly launch doesn't recognize, write a minimal Dockerfile yourself or rely on Cloud Native Buildpacks. On Fly.io, once a Dockerfile exists in the project, every subsequent run picks it up automatically.

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