Fly.io

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Dotenvy can be used when deploying an app to Fly.io; other hosting services may require customizations similar to the ones demonstrated here. Following the instructions on the Phoenix page will get you most of the way there, but you also need to modify the Dockerfile so it copies over your envs/ directory: the .env files must be available to the release command. The default Fly.io Dockerfile needs a small alteration to ensure that your .env files are along for the ride.

Dockerfile

A Dockerfile gets generated when you setup your app (it is ultimately generated from mix phx.gen.release --docker). The Dockerfile is ultimately what is responsible for running mix release, so you need to ensure that all of your files and folders are copied into the container.

In the examples provided in this documentation, .env files are stored in a directory named envs/. If that's the directory housing your .env files, then you will need to ensure that folder gets copied into the Docker container so it can properly build the release. To do this, you will need to add a line COPY envs envs before the Run mix release command.

# ... existing Docker stuff...

# Compile the release
RUN mix compile

# Changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/

COPY rel rel

# <---- make the directory of envs available to the release!!!
COPY envs envs

RUN mix release

# ... existing Docker stuff cont'd...

Once that line is there, then running fly deploy and other flyctl commands should be able to build the release and include the .env files as expected. You can test the standard GitHub workflows by pushing changes to main.

The importance of path names

Because your app executes inside a Docker container, it is really important to use Path.absname/2 to reference your .env files. The reason we mention this ad nauseam in these docs is because it's so critical. Different release strategies may set the RELEASE_ROOT variable with or without a trailing slash, and that can mean that Dotenvy can't find your .env files. It can be helpful to specify the :require_files option to Dotenvy.source/2 just to help alert you when the paths may be pointing to the wrong place, e.g.

env_dir_prefix = System.get_env("RELEASE_ROOT") || Path.expand("./envs")

source!(
  [
    Path.absname(".env", env_dir_prefix),
    Path.absname(".#{config_env()}.env", env_dir_prefix),
    Path.absname(".#{config_env()}.overrides.env", env_dir_prefix),
    System.get_env()
  ],
  require_files: [Path.absname(".env", env_dir_prefix)]
)

Environment Variables

Fly.io sets a handful of environment variables when it deploys an app (which you can inspect by running System.get_env() from an iex shell). The following are the most significant environment variables used in the config files generated by the Dotenvy Generators:

  • RELEASE_ROOT
  • DATABASE_URL
  • PHX_HOST
  • PORT
  • PHX_SERVER
  • DNS_CLUSTER_QUERY
  • SECRET_KEY_BASE

See the Phoenix page for getting a Phoenix app to run using Dotenvy.