Using the Fly Docker registry lets you deploy Docker containers that will be compiled as a Firecracker VM and run in locations around the world.

Fly offer a number of ways to build and deploy apps. For many frameworks you can run fly launch and it will detect the framework and configure a container for you. For others you can pass it a Dockerfile which will be built and deployed. But you can also push your own images to a Docker registry and deploy them to Fly.

Today I figured out how to use Fly's own registry to deploy an app.

Tagging images for the Fly registry

Fly's registry is called To use it, you need to tag your Docker images with a tag that begins with that string.

Every Fly app gets its own registry subdomain. You can create apps in a number of ways, but the easiest is to use the Fly CLI:

flyctl apps create datasette-demo

Fly app names must be globally unique across all of Fly - you will get an error if the app name is already taken.

You can create an app with a random, freely available name using the --generate-name option:

~ % flyctl apps create --generate-name
? Select Organization: Simon Willison (personal)
New app created: rough-dew-1296

Now that you have an app name, you can tag your Docker image using:

If you are building an image using Docker on your machine, you can run this command in the same directory as your Dockerfile:

docker build -t .

Pushing images to the registry

In order to push your image to Fly, you will first need to authenticate.

The flyctl auth docker command will do this for you:

~ % flyctl auth docker
Authentication successful. You can now tag and push images to{your-app}

This works by hooking into Docker's own authentication mechanism. You can see what it has done by looking at your ~/.docker/config.json file. Mine looks like this:

  "auths": {
    "": {
      "auth": "... secret token here ..."
  "experimental": "disabled",
  "stackOrchestrator": "swarm"

Now you can push your image to the registry like this:

docker push

Deploying an image

Now that your image is pushed, you can deploy an instance of it like this:

flyctl deploy --app datasette-demo \

A few seconds later your app will be running at:


Created 2022-05-21T19:33:19-07:00 · Edit