Docker: Difference between revisions

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[https://www.docker.com/ Docker] is a platform for building, packaging, and distributing applications inside containers. Containers bundle an application's code, configurations, and dependencies into a single object that runs consistently across different computing environments. Docker works well with NixOS through the virtualization module.<ref>https://www.docker.com/resources/what-container/</ref>

== Installation ==

==== Shell ====

To temporarily use Docker in a shell environment, you can run:

<syntaxhighlight lang="bash">

nix-shell -p docker

</syntaxhighlight>

This will provide a shell with Docker CLI available, but note that the Docker daemon will not be running. For full functionality, you'll need a system-level installation.

==== System setup ====

To install Docker on NixOS, add the virtualization.docker module to your system configuration at <code>/etc/nixos/configuration.nix</code>:<ref>https://nixos.org/manual/nixos/stable/options#opt-virtualisation.docker.enable</ref>

<syntaxhighlight lang="nix">

# In /etc/nixos/configuration.nix

==== Basic ====

The basic Docker configuration on NixOS includes several options you can set in your <code>configuration.nix</code> file:

<syntaxhighlight lang="nix">

virtualisation.docker = {

==== Advanced ====

For more advanced configuration, you can customize Docker daemon options and networking:

<syntaxhighlight lang="nix">

virtualisation.docker = {

};

</syntaxhighlight>

== Docker Compose ==

Currently, there are two options to use Docker Compose with NixOS: Arion or Compose2Nix.

With Arion, you can specify most Docker Compose options in Nix Syntax, and Arion will generate a <code>docker-compose.yml</code> file internally. The result is a systemd service that starts and stops the container.  

=== Arion ===

[https://docs.hercules-ci.com/arion/ Arion] is created for running Nix-based projects in Docker Compose. It uses the NixOS module system for configuration, it can bypass <code>docker build</code> 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 your NixOS configuration:

=== Compose2Nix ===

With [https://github.com/aksiksi/compose2nix compose2nix] you can generate [https://search.nixos.org/options?query=virtualisation.oci-containers oci-containers] config from a <code>docker-compose.yaml</code>.  

==== Install ====

nix shell github:aksiksi/compose2nix

compose2nix -h

compose2nix = {

   url = "github:aksiksi/compose2nix";

   inputs.nixpkgs.follows = "nixpkgs";

};

environment.systemPackages = [

   inputs.compose2nix.packages.x86_64-linux.default

==== Usage ====

After you have installed <code>compose2nix</code>, you can run <code>compose2nix</code> in the directory with your <code>docker-compose.yml</code>, which will output a <code>docker-compose.nix</code>.

compose2nix -inputs input.yml -output output.nix -runtime docker

== Tips and tricks ==

=== Docker on btrfs ===

If you use the [[btrfs]] file system, you might need to set the {{nixos:option|virtualisation.docker.storageDriver|storageDriver}} option:

<syntaxhighlight lang="nix">

virtualisation.docker.storageDriver = "btrfs";

=== Rootless Docker ===

Rootless Docker lets you run the Docker daemon as a non-root user for improved security. Set the <code>rootless</code> option [[#Advanced|as shown above]]. The <code>setSocketVariable</code> option adds the <code>DOCKER_HOST</code> variable pointing to your rootless Docker instance.

After enabling rootless mode, Docker can be started with:

==== Building a docker image with nixpkgs ====

There is an entry for [https://nixos.org/nixpkgs/manual/#sec-pkgs-dockerTools dockerTools] in the Nixpkgs manual for reference. In the linked page, they give the following example config:

<syntaxhighlight lang="nix">

==== Reproducible image dates ====

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

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

==== Calculating the sha256 for a pulled Docker image ====

The <code>sha256</code> argument of the <code>dockerTools.pullImage</code> 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 <code>dockerTools.pullImage</code> function.

For instance, the SHA of the following image

==== Directly Using Nix in Image Layers ====

Instead of copying Nix packages into Docker image layers, Docker can be configured to directly utilize the <code>nix-store</code> by integrating with [https://github.com/pdtpartners/nix-snapshotter nix-snapshotter].

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

=== Using Podman as an alternative ===

<syntaxhighlight lang="nix">

# Enable Podman in configuration.nix

=== Changing Docker Daemon's Data Root ===

By default, the Docker daemon stores images, containers, and build context on the root file system. To use a different storage location, specify a new <code>data-root</code> in your configuration:

<syntaxhighlight lang="nix">

virtualisation.docker.daemon.settings = {

=== Docker Containers as systemd Services ===

You can run Docker containers as systemd services using the <code>oci-containers</code> module:

<syntaxhighlight lang="nix">

virtualisation.oci-containers = {

==== Usage ====

Unless otherwise specified, NixOS uses Podman to run OCI containers. Note that these are '''user-specific''', so running commands with or without sudo can change your output.

# podman ps

# podman restart hackagecompare

# podman ls

# podman rm hackagecompare

# podman rmi c0d9a5f58afe

# podman pull chrissound/hackagecomparestats-webserver:latest

# podman exec -ti $ContainerId /bin/sh

</syntaxhighlight>

===== Exposing ports from the host =====

If you have a service running on the host that you want to connect to from the container, you could try connecting to the hostname <code>host.containers.internal</code> (or <code>host.docker.internal</code> for podman), but this might require additional networking setup

===== Exposing sockets from the host =====

If you have a service running on the host that exposes a socket, such as mariadb, you can also expose that socket to the container instead. You'll want to expose the folder the socket is in as a volume - so:

<syntaxhighlight lang="bash">

       volumes = [

       ];

</syntaxhighlight>

to provide access to <code>/var/run/mysqld/mysqld.sock</code>

=== Running the docker daemon from nix-the-package-manager - not NixOS ===

This is not supported. You're better off installing the docker daemon [https://docs.docker.com/engine/install/ "the normal non-nix way"].

See the discourse discussion: [https://discourse.nixos.org/t/how-to-run-docker-daemon-from-nix-not-nixos/43413 How to run docker daemon from nix (not NixOS)] for more.

==== Cannot connect to the Docker daemon ====

If you encounter errors connecting to the Docker daemon, check that:

- The Docker service is running: `systemctl status docker`

- Your user is in the docker group: `groups | grep docker`

- You've logged out and back in after adding your user to the docker group

==== Storage space issues ====

When Docker uses too much disk space:

<syntaxhighlight lang="bash">

# Remove unused containers, networks, images, and volumes

==== Network conflicts ====

Docker's default subnet (`172.17.0.0/16`) might conflict with your existing network. Configure a different subnet in your `configuration.nix`:

<syntaxhighlight lang="nix">

virtualisation.docker.daemon.settings = {

=== Cannot connect to public Wi-Fi, when using Docker ===

When connecting to a public Wi-Fi, where the login page's IP-Address is within the Docker network range, accessing the Internet might not be possible. This has been reported when trying to connect to the WIFIonICE of the Deutsche Bahn (DB). They use the <code>172.18.x.x</code> address range.

This can be resolved by changing the default address pool that Docker uses.