How to use docker containers in a snap application

This guide illustrates how to build a snap application that leverages the docker snap maintained and supported by Canonical, to bring containers to IoT, server-side or desktop applications.

Digression about security aspects

There are two possible avenues, one of which is explored in this guide:

1. Build a complete OCI container runtime into an application snap.
2. Integrate with an external snap, like docker, without bundling it. This is the approach we choose in this guide.

The key thing to understand is that the equivalent functionality is nearly the same, there are significant security differences between the two approaches.

While anyone can rebuild the docker snap from source, not everyone is immediately trusted with the privileged interfaces that are required for the docker snap to function. Due to the nature of containers, some of the permissions granted to the docker snap allow it to bypass elements of the security sandbox. This is acceptable because the publisher is trusted and because applications running inside such containers are further isolated with the sandbox built by docker itself.

It is therefore a lot easier to create and publish a snap-based application that talks to docker over the docker socket or uses docker command-line interface than to build the runtime into the snap directly.

Application skeleton

Let's look at a skeleton snapcraft.yaml file. Parts that are not relevant to the topic are elided. A complete, working file is found in the corresponding repository.

base: core22
confinement: strict
apps:
  hello-docker:
    command: usr/bin/hello-docker
    environment:
      PATH: $SNAP/docker-snap/bin:$PATH
    plugs:
      - docker
      - docker-executables
plugs:
  docker:
    label: Access to the docker communication socket
  docker-executables:
    label: Access to the docker command-line utilities 
    interface: content
    content: docker-executables
    target: $SNAP/docker-snap
    default-provider: docker

Let's review the key parts here. We are looking at a strictly-confined snap package. This type of package is portable across environments, working equally in IoT-centric ubuntu-core as well as on most commonly used desktop and server distributions.

The snap has one application, the hello-docker application. This application has uses two snap interface plugs, one called docker and the other one called docker-executables. The plugs are defined in more detail and we can see that while docker is one of the built-in interfaces, docker-executables is an example of the content interface. The content interface is further specified to refer to content of type docker-executables. This is important as it has to match what is provided by the docker snap available in the store.

Plugs of the content interface have the target attribute which defines where the corresponding content is made available. In this case it is the directory docker-snap inside the read-only image of our application snap. It is important that our application snap contains an empty directory with the same name, so that when the docker snap is installed and the interfaces are connected, we can get access to docker command-line tools.

The application has an environment variable that causes us to look for docker executables in the $SNAP/docker-snap/bin sub-directory. This sub-directory will only exist when the content interface is connected.

Lastly the default-provider field tells snapd that if the user does not have any snap with a compatible interface installed, then upon installation of our snap, the docker snap is automatically installed and connected.

Principle of operation

Our application snap should cleanly build with snapcraft and install with sudo snap install --dangerous ./hello-docker_1_all.snap. On first install you may see that the core22 base snap and the docker snaps are automatically installed.

After install you will also need to ensure that there is a connection relationship with your snap's docker-executables plug and the docker snap's docker-executables slot:

$ snap connect hello-docker:docker-executables docker:docker-executables

Let's explore the runtime environment that our application runs in:

$ snap run --shell hello-docker
$ cd $SNAP
$ ls docker-snap/bin

If everything is working fine, we should see the docker command-line tool, among other executables. Since it is on $PATH we should be able to use docker in our application script for whatever purpose we want.

Caveats

• Discuss where container storage is
• Discuss consequence of multiple snaps using the single docker snap