Dockerfiles and fly.toml

Once you have completed running fly launch you have some new files, most notably a Dockerfile and a fly.toml file. For many applications you are ready to deploy. But before you do, scan the following list to see if any of these common situations apply to you, and how to proceed.

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Your application is unlikely to stay the same forever. Perhaps you’ve updated to a new version of Ruby, bundler, node, or other package. Or added a gem which has system dependencies. When this occurs, you will need to update your Dockerfile to match. In most cases, all you need to do is rerun the generator:

bin/rails generate dockerfile

The generator will remember the options you selected before (these are stored in config/dockerfile.yml). If you need to change a boolean option, add or remove a no- prefix before the option name.

If you have made hand edits to your Dockerfile you may want to take advantage of the option to diff the changes before they are applied.

Custom Packages

The Dockerfile generator for Rails attempts to detect common dependencies and handle them for you. If you come across a dependency that may be useful to others, please consider opening up an issue or a pull request.

You may have needs beyond what is automatically detected. Most official Ruby docker images are based on Debian bullseye, and there are a large number of packages available to be installed in this manner.

An example adding basic kernel and network monitoring packages from this list:

bin/rails generate dockerfile --add procps net-tools traceroute iputils-ping

Using Sqlite3

Every time you deploy you will start out with a fresh image. If your database is on that image, it too will start fresh which undoubtedly is not what you want.

The solution is to create a Fly Volume.

Once you have created a volume, you will need to set the DATABASE_URL environment variable to cause Rails to put your database on that volume. The result will be the following lines in your fly.toml file:

  DATABASE_URL = "sqlite3:///mnt/volume/production.sqlite3"

  source = "sqlite3_volume"
  destination = "/mnt/volume"

Adjust the name of the source to match the name of the volume you created.

Out of Memory

RAM is a precious commodity - both to those on Hobby plans who want to remain within or near the free allowances, and to apps that want to scale to be able to handle a large number of concurrent connections.

Both fullstaq and jemalloc are used by many to reduce their memory footprint. As every application is different, test your application to see if either are appropriate for you. Enabling one or both can be done by regenerating your Dockerfile and specifying the appropriate option(s):

bin/rails generate dockerfile --fullstaq --jemalloc

At some point you may find that you need more memory. There are two types: real and virtual. Real is faster, but more expensive. Virtual is slower but free.

To scale your app to 1GB of real memory, use:

fly scale memory 1024

To allocate 1GB of swap space for use as virtual memory, add the following to your fly.toml:

swap_size_mb = 1024


If your application involves multiple servers, potentially spread across a number of regions, you will want to prepare your databases once per deploy not once per server.

Regenerate your Dockerfile specifying that you no longer want the prepare step there:

bin/rails generate dockerfile --no-prepare

Next, add a deploy step to your fly.toml:

  release_command = "bin/rails db:prepare"

Shelling in

Fly provides the ability to ssh into your application, and it would be convenient to run things like the Rails console in one line:

fly ssh console --pty -C '/rails/bin/rails console'

To enable bin/rails commands to be run in this manner, adjust your deployed binstubs to set the current working directory:

bin/rails generate dockerfile --bin-cd

Build speeds

The Dockerfile you were provided will only install gems and node modules if files like Gemfile and package.json have been modified. If you are finding that you are doing this often and deploy speed is important to you, turning on build caching can make a big difference. And if your Rails application makes use of node.js, installing gems and node packages in parallel can reduce build time. You can regenerate your Dockerfile to enable one or both:

bin/rails generate dockerfile --cache --parallel

Runtime performance

Ruby images starting with 3.2 include YJIT but disabled. You can enable YJIT using:

bin/rails generate dockerfile --yjit

Testing Locally

If you have Docker installed locally, you can test your applications before you deploy them by running the following commands:

bin/rails generate dockerfile --compose
export RAILS_MASTER_KEY=$(cat config/master.key)
docker compose build
docker compose up

Windows PowerShell users will want to use the following command instead of export:

$Env:RAILS_MASTER_KEY = Get-Content 'config\master.key'