Building a constellation of images with Earthly

Published on , 1237 words, 5 minutes to read

What if building container images was actually a graph?

An image of A film-emulated picture of a the sky on one side, and treetops on the other. There is heavy saturation and a slight hint of film grain.
A film-emulated picture of a the sky on one side, and treetops on the other. There is heavy saturation and a slight hint of film grain. - Photo by Xe Iaso, iPhone 15 Pro Max

Docker is the universal package format of the Internet. It allows you to ship an application and all of its dependencies in one unit so that you can run it without worrying about dependencies on the host machine breaking your app. It's quickly become the gold standard for how to package and deploy applications, and it's not hard to see why.

However, the main way you build a Docker image is with the docker build command, which takes a Dockerfile in the directory you specify on the command line and then builds an image from that. This works great for single-component applications, or even facets of a larger monorepo, but it falls short when you have something like a monorepo written in Go that has multiple components that need to be built and then packaged into separate images.

I have two big "monorepos" of side projects and the like that I want to deploy as Docker images. One is my blog, and the other is my /x/ experimental monorepo. Both of these projects have multiple components that need to be built and packaged into separate Docker images, and I've been struggling to find a way to do this that was as good as the previous setup.

When I was working with a coworker on something recently, I was pointed to Earthly. Earthly is a unique form of violence, it's effectively a bastard child of Make and Docker that was raised by a team of people who really care about developer ergonomics. The best way to think about Earthly is that it's a build system that just happens to execute every step in a container and you can fossilize artifacts or images out of the build process.

Under the hood, Docker has started to use BuildKit to make images. This effectively transforms a Dockerfile into a graph of steps that can be executed in parallel. Consider this Dockerfile:

FROM golang:1.22 AS builder
COPY . .
RUN mkdir -p /app/bin && go build -o /app/bin/myapp ./cmd/myapp

FROM nodejs AS frontend
COPY . .
RUN npm install && npm run build

FROM ubuntu:24.04 AS runner
COPY --from=builder /app/bin/myapp /app/bin/myapp
COPY --from=frontend /src/build /app/static
CMD ["/app/bin/myapp"]

This effectively turns a build into a graph like this:

Dockerfile graph

The builder and frontend stages can be built in parallel, but the runner stage needs to wait until both of them are done before it can be built. This is a simple example, but it shows how you can have multiple components that need to be built and then packaged into a single image.

What if you have multiple images though? That's where Earthly comes in. Earthly builds on top of BuildKit to allow you to define a series of targets that can be built in parallel, and then it builds them in the most efficient way possible. It's like Make, but for Docker images.

My blog's backend

My blog is an unfortunately complicated project, it wasn't intended to be that way, it sorta organically grew this way after a decade or so. It's a Go project that requires on a few components:

After breaking everything down into the components and inputs, I came up with the following flow:

Xesite build graph

Going from left to right, the inputs are:

These are then passed through to pull and build the components and their dependencies. The +patreon and +xesite targets are the final images that are built from the components. The +xesite target is a bit weird in that we need to copy the Iosevka Iaso font files and the Dhall binary into the image so that the blog can use them (it will panic at runtime if it can't find them).

These two targets are then pushed to the GitHub Container Registry so that they can be pulled down and run on

Cadey is coffee

At the time of writing, is my employer. I'm using to run my blog. I'm not just shilling it for the sake of shilling it. I was a user before I was an employee, and I'm still a user now that I'm an employee. It's a great platform and I love it. If the platform wasn't great, I wouldn't be using it.

Oh, as a side note, when you're trying to build multiple images at once from CI, you need to make an all target or similar that depends on all of the images you want to build. This is because Earthly can only build one target at a time.

    BUILD --platform=linux/amd64 +xesite
    BUILD --platform=linux/amd64 +patreon-saasproxy

You can then chuck this into GitHub Actions:

- name: Build and push Docker image
  id: build-and-push
  run: |
    earthly --ci --push +all

The --ci flag sets some options that help Earthly work better in a CI environment. It's not strictly necessary, but it's probably a good idea to use it.

The impact

The difference between these two flows is subtle but staggering. Building my blog's backend with the old flow could take up to 10 minutes. Building my blog's backend with Earthly takes tens of seconds. The old flow produced a 734 MB image with a bunch of extraneous dependencies (even though that should be mathematically impossible). The new flow shits out a 262 MB image that has only what is required to run the blog.

Not to mention the developer ergonomics of using Earthly. With Earthly I can build and push my images in one Go. I don't even run into the Dockerfile landmine of forgetting to run docker build -t before running docker push. I can't tell you how many times I've done that and had to dig up the image reference from docker images to manually tag and push it.

Earthly is exactly what I needed. I'm going to adopt it as my Docker image build system of choice.

The only downside is them adding advertisements for their SaaS product in all of my build outputs:

🛰️ Reuse cache between CI runs with Earthly Satellites! 2-20X faster than without cache. Generous free tier

I get why they're doing this, it's really hard to make money off of developer tooling like this. Developers are both extremely well paid and notoriously cheap. I'm not going to fault them for trying to make money off of their product. I just wish I could turn it off.

Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Tags: earthly, docker, xesite