Building container images with Nix

Dockerfiles are de facto standard for creating container images. In this article I highlight some issues with this approach and I propose building container images with Nix.

Container images

Many compute engines adopted container images as an interface between your code and their runtime platform. You package your application in a self-contained bundle and hand it over to your compute engine of choice for execution. The compute engine’s runtime system creates one or many container instances based on the provided image and runs your application using some form of isolation.

Typically the application doesn’t have access to any software packages installed on the physical computer where it runs. Therefore, the application can only run correctly in a container if all of its software dependencies are also packaged in the same image.

Dockerfile

The canonical way of creating container images is via a Dockerfile. For example, let’s create an image for a web application written in Python:

FROM alpine:3.5  # ①

RUN apk add --update py2-pip  # ②

RUN pip install --upgrade pip  # ③

COPY requirements.txt /usr/src/app/  # ④
RUN pip install --no-cache-dir -r \
    /usr/src/app/requirements.txt  # ⑤
COPY app.py /usr/src/app/  # ⑥
COPY templates/index.html /usr/src/app/templates/  # ⑦

EXPOSE 5000
CMD ["python", "/usr/src/app/app.py"]

This is a short, working example which you can use to deploy a non-trivial web application. But if you look closely, there are some issues with this Dockerfile. Let’s see them, line by line:

  1. The FROM keyword indicates that we’re starting from a base image. In other words, we define a dependency on a binary blob. You may not know what programs and libraries the selected base image contains. In this specific case, the Alpine Linux Docker images are described in this repository.

  2. The apk package manager, part of the base image, installs some version of the Python interpreter which is required to run our application. Because we don’t specify the exact version, every time you build this image you may install different versions.

  3. We install pip which we’ll need at subsequent build steps, but not when the application is running. The problem of the unpredictable versions, mentioned in the previous item, applies here as well.

  4. We copy the requirements.txt to the image. This also only required for building the image.

  5. The pip package manager installs the run-time dependencies of our application. If requirements.txt specifies exact version numbers we know exactly which packages will be in the final image.

  6. We copy the application, which is only a single file in this case.

  7. We copy an HTML template, also required during run-time.

Each line transfers some component into the final container image. However, the Dockerfiles imperative language fails to capture precisely our application’s dependencies:

We find Dockerfiles with such structure used by many software projects. I took this section’s example from a Docker for Beginners repository. In the next section we’ll see how novice users write their Dockerfiles.

Dockerfile from scratch

The ideal container image contains the application and the application’s dependencies and nothing else.

Kelsey Hightower shows how to build such a container image for an application called weather-data-collector using the following Dockerfile:

FROM scratch
COPY weather-data-collector .
ENTRYPOINT ["/weather-data-collector"]

This copies the application’s locally built executable on the image’s filesystem. The image contains no shell, no package manager, no additional libraries. Both image size and its the attack surface is minimal. This technique does work and creates an ideal container image and does solve all the issues raised in the previous section.

There’s a limitation though: this Dockerfile assumes that the application is built as a single, statically linked binary.

Also, by reading this Dockerfile we don’t know anything about provenance of the application’s binary: which build commands were executed, which tool chain version is used, what additional libraries were installed before building. To remedy this it’s common to use multi-stage Dockerfiles. The first stage prepares the build environment and builds the application’s binary. Then, during second stage, the binary is copied into an independent layer.

Multi-stage builds may work well for you if you always run the application in containers, even during development. In my experience, however, the build instructions in the Dockerfile quickly become the duplicate of the project’s native build instructions.

Blame your build system

I don’t believe that writing a Dockerfile is inherently bad. I think the underlying problem is that our build systems do a poor job of capturing our application’s dependencies. The various Dockerfile tricks are just workarounds for the absence of proper dependency management.

Take a look at any non-trivial project’s build instructions. I bet you’ll find a getting started guide with a long list of tools and libraries to install before you can invoke the project’s build system.

This is admitting that the project’s dependency lists are incomplete and you need out-of-band manipulations such as installing a package or downloading something else from the Internet.

Container images with Nix

Nix was explicitly designed to capture all dependencies of a software component. This makes it a great fit for building ideal container images which contain the application, the application’s dependencies and nothing else.

To demonstrate, here’s an example which packages the HTML pages of this blog and a webserver into a container image.

containerImage = pkgs.dockerTools.buildLayeredImage
  {
    name = "thewagner.net";
    contents = [ pkgs.python3 htmlPages ];
    config = {
      Cmd = [
        "${pkgs.python3}/bin/python" "-m" "http.server" 8000
        "--directory" "${htmlPages}"
      ];
      ExposedPorts = {
        "8000/tcp" = { };
      };
    };
  };

You can run the built image yourself:

docker run -p 8000:8000 wagdav/thewagner.net

If you visit http://localhost:8000 you can read my articles served from the locally running container.

This snippet calls the function buildLayeredImage from the Nix Packages Collection to build the container image. The function’s arguments define the image’s name, contents and its configuration according to the OCI specification. The function returns a derivation which builds a container image containing the Python interpreter, for the webserver, and the static pages of my blog.

In short, you list the components you want in the image and Nix copies those and their dependencies into the image archive. Perhaps this summary sounds underwhelming, but proper dependency management renders the building of the container images conceptually simpler. Instructions in a Dockerfile are analogous to a shell script provisioning a freshly installed computer. The Nix expression looks like creating a compressed archive specifying a list of files.

To use buildLayeredimage you must have a Nix expression to build your application. The Nixpkgs manual has a long section on building packages for various programming languages.

Concretely, for the example showed in this section, you can find the complete definition of htmlPages here. Everything is built from source and the pkgs attribute set points to a specific commit of the Nix Packages collection. The image is reproducible: no software version changes between builds unless explicitly updated.

As a bonus, this image is automatically built by GitHub Actions every time I push new content in the source repository. And this works without fiddling with QEMU, BuildX and Docker or using a special remote Docker setup.

Summary

Dockerfiles, when used naively, yield bloated container images with non-deterministic content. The various techniques such as building static executables and using multi-staged builds are just workarounds for the absence of a programming language independent build system which is capable of capturing all dependencies of an application.

Using Nix and the Nix Packages Collection it’s possible to build minimal, reproducible container images with a few lines of code which run locally and on any hosted build automation system.

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