Docker is a utility to pack, ship and run any application as a lightweight container.

Docker on NixOS

Installation

To install docker, add the following to your your NixOS configuration:

virtualisation.docker.enable = true;

More options are available.

Adding users to the docker group will provide them access to the socket:

users.users.<myuser>.extraGroups = [ "docker" ];

If you prefer, you could achieve the same with this:

users.extraGroups.docker.members = [ "username-with-access-to-socket" ];

If you're still unable to get access to the socket, you might have to re-login or reboot.

Warning: Beware that the docker group membership is effectively equivalent to being root!
Consider using rootless mode below.

Note: If you use the btrfs filesystem, you might need to set the storageDriver option:

virtualisation.docker.storageDriver = "btrfs";

Rootless docker

To use docker in rootless mode, you can activate the rootless option:

virtualisation.docker.rootless = {
  enable = true;
  setSocketVariable = true;
};

The setSocketVariable option sets the DOCKER_HOST variable to the rootless Docker instance for normal users by default.

Changing Docker Daemon's Data Root

By default, the Docker daemon will store images, containers, and build context on the root filesystem.

If you want to change the location that Docker stores its data, you can configure a new data-root for the daemon by setting the data-root property of the virtualisation.docker.daemon.settings.

virtualisation.docker.daemon.settings = {
  data-root = "/some-place/to-store-the-docker-data";
};

Docker Containers as systemd Services

To make sure some docker containers are running as systemd services, you can use 'oci-containers':

virtualisation.oci-containers = {
  backend = "docker";
  containers = {
    foo = {
      # ...
    };
  };
};

See https://mynixos.com/options/virtualisation.oci-containers.containers.%3Cname%3E for further options

Creating images

Building a docker image with nixpkgs

There is an entry for dockerTools in the nixpkgs manual for reference. In the linked page they give the following example config:

buildImage {
  name = "redis";
  tag = "latest";

  fromImage = someBaseImage;
  fromImageName = null;
  fromImageTag = "latest";

  copyToRoot = pkgs.buildEnv {
    name = "image-root";
    paths = [ pkgs.redis ];
    pathsToLink = [ "/bin" ];
  };

  runAsRoot = ''
    #!${pkgs.runtimeShell}
    mkdir -p /data
  '';

  config = {
    Cmd = [ "/bin/redis-server" ];
    WorkingDir = "/data";
    Volumes = { "/data" = { }; };
  };

  diskSize = 1024;
  buildVMMemorySize = 512;
}

More examples can be found in the nixpkgs repo.

Also check out the excellent article by lethalman about building minimal docker images with nix.

Reproducible image dates

The manual advises against using created = "now", as that prevents images from being reproducible.

An alternative, if using flakes, is to do created = builtins.substring 0 8 self.lastModifiedDate, which uses the commit date, and is therefore reproducible.

How to calculate the `sha256` of a pulled image

The sha256 argument of the dockerTools.pullImage function is the checksum of the archive generated by Skopeo. Since the archive contains the name and the tag of the image, Skopeo arguments used to fetch the image have to be identical to those used by the dockerTools.pullImage function.

For instance, the sha of the following image

pkgs.dockerTools.pullImage{
  imageName = "lnl7/nix";
  finalImageTag = "2.0";
  imageDigest = "sha256:632268d5fd9ca87169c65353db99be8b4e2eb41833b626e09688f484222e860f";
  sha256 = "1x00ks05cz89k3wc460i03iyyjr7wlr28krk7znavfy2qx5a0hfd";
};

can be manually generated with the following shell commands

skopeo copy docker://lnl7/nix@sha256:632268d5fd9ca87169c65353db99be8b4e2eb41833b626e09688f484222e860f docker-archive:///tmp/image.tgz:lnl7/nix:2.0

nix-hash --base32 --flat --type sha256 /tmp/image.tgz

1x00ks05cz89k3wc460i03iyyjr7wlr28krk7znavfy2qx5a0hfd

Directly Using Nix in Image Layers

Instead of copying Nix packages into Docker image layers, Docker can be configured to directly utilize the nix-store by integrating with nix-snapshotter.

This will significantly reduce data duplication and the time it takes to pull images.

Docker Compose with Nix

Arion is created for running Nix-based projects in Docker Compose. It uses the NixOS module system for configuration, it can bypass docker build and lets you use dockerTools or use the store directly in the containers. The images/containers can be typical dockerTools style images or full NixOS configs.

To use Arion, you first need to add its module to you NixOS configuration:

modules = [ arion.nixosModules.arion ];

After that you can access its options under

virtualisation.arion = {}

A config for a simple container could look like this:

virtualisation.arion = {
  backend = "docker";
  projects = {
    "db".settings.services."db".service = {
      image = "";
      restart = "unless-stopped";
      environment = { POSTGRESS_PASSWORD = "password"; };
    };
  };
};

Using Nix in containers

While dockerTools allows to build lightweight containers, it requires nix to be installed on the host system. An alternative are docker images with nix preinstalled:

nixos/nix (official)
nixpkgs/nix (built from https://github.com/nix-community/docker-nixpkgs)

NixOS can be run in containers using Arion.