Building and running Docker images

Docker is a set of tools and services used to build, manage and deploy containers.

As many cloud platforms offer Docker-based container hosting services, creating Docker containers for a given service is a common task when building reproducible software. In this tutorial, you will learn how to build Docker containers using Nix.

Prerequisites

We assume you have both Nix and Docker installed. Docker is available in nixpkgs, which is the preferred way to install it on NixOS. However, you can also use the native Docker installation of your OS, if you are on another Linux distribution or MacOS.

Build your first container

Nixpkgs provides dockerTools to create Docker images:

{ pkgs ? import <nixpkgs> { system = "x86_64-linux"; } }:

pkgs.dockerTools.buildImage {
  name = "hello-docker";
  config = {
    Cmd = [ "${pkgs.hello}/bin/hello" ];
  };
}

We call the dockerTools.buildImage and pass in some parameters:

  • a name for our image

  • the config including the command Cmd that should be run inside the container once the image is started. Here we reference the GNU hello package from nixpkgs and run its executable in the container.

Save this in hello-docker.nix and build it:

$ nix-build hello-docker.nix
these derivations will be built:
  /nix/store/qpgdp0qpd8ddi1ld72w02zkmm7n87b92-docker-layer-hello-docker.drv
  /nix/store/m4xyfyviwbi38sfplq3xx54j6k7mccfb-runtime-deps.drv
  /nix/store/v0bvy9qxa79izc7s03fhpq5nqs2h4sr5-docker-image-hello-docker.tar.gz.drv
warning: unknown setting 'experimental-features'
building '/nix/store/qpgdp0qpd8ddi1ld72w02zkmm7n87b92-docker-layer-hello-docker.drv'...
No contents to add to layer.
Packing layer...
Computing layer checksum...
Finished building layer 'hello-docker'
building '/nix/store/m4xyfyviwbi38sfplq3xx54j6k7mccfb-runtime-deps.drv'...
building '/nix/store/v0bvy9qxa79izc7s03fhpq5nqs2h4sr5-docker-image-hello-docker.tar.gz.drv'...
Adding layer...
tar: Removing leading `/' from member names
Adding meta...
Cooking the image...
Finished.
/nix/store/y74sb4nrhxr975xs7h83izgm8z75x5fc-docker-image-hello-docker.tar.gz

The image tag (y74sb4nrhxr975xs7h83izgm8z75x5fc) refers to the Nix build hash and makes sure that the Docker image corresponds to our Nix build. The store path in the last line of the output references the Docker image.

Run the container

To work with the container, load this image into Docker’s image registry from the default result symlink created by nix-build:

$ docker load < result
Loaded image: hello-docker:y74sb4nrhxr975xs7h83izgm8z75x5fc

You can also use the store path to load the image in order to avoid depending on the presence of result

$ docker load < /nix/store/y74sb4nrhxr975xs7h83izgm8z75x5fc-docker-image-hello-docker.tar.gz
Loaded image: hello-docker:y74sb4nrhxr975xs7h83izgm8z75x5fc

Even more conveniently, you can do everything in one command. The advantage of this approach is that nix-build will rebuild the image if there are any changes and pass the new store path to docker load:

$ docker load < $(nix-build hello-docker.nix)
Loaded image: hello-docker:y74sb4nrhxr975xs7h83izgm8z75x5fc

Now that you have loaded the image into Docker, it is time to run it:

$ docker run -t hello-docker:y74sb4nrhxr975xs7h83izgm8z75x5fc
Hello, world!

Working with Docker images

A general introduction to working with Docker images is not part of this tutorial. The official Docker documentation is a much better place for that. Note that when you build your Docker images with Nix, you will probably not write a Dockerfile as Nix replaces the Dockerfile functionality within the Docker ecosystem.

Nonetheless, understanding the anatomy of a Dockerfile may still be useful to follow along how Nix replaces each of its functions. Using the Docker CLI, Docker Compose, Docker Swarm or Docker Hub on the other hand may still be relevant depending on your use case.

Next steps

More details on how to use dockerTools can be found in the nixpkgs manual.

You will also find more advanced examples of Docker images built with Nix in the examples file on nixpkgs.