Using Docker in Earthly

This guide walks through using Docker commands in Earthly.

Basic usage

In order to use Docker commands (such as docker run), Earthly makes available isolated Docker daemons which are started and stopped on-demand. The reason for using isolated instances of Docker daemons is such that no pre-existing Docker state (e.g. images, containers, networks, volumes) can influence the way the build executes. This allows Earthly to achieve high degrees of reproducibility.

Here is a quick example of running a hello-world docker container via docker run in Earthly:

FROM earthly/dind:alpine
WITH DOCKER --pull hello-world
RUN docker run hello-world

Let's break it down.

FROM earthly/dind:alpine inherits from an Earthly-supported docker-in-docker (dind) image. This is recommended, because WITH DOCKER requires all the Docker binaries (not just the client) to be present in the build environment.

WITH DOCKER ... END starts a Docker daemon for the purpose of running Docker commands against it. At the end of the execution, this also terminates the daemon and permanently deletes all of its data (e.g. daemon cached images).

--pull hello-world pulls the image hello-world from the Docker Hub. This option could have been replaced with the more traditional docker pull hello-world. However, the Earthly variant additionally stores the image in the Earthly cache, so that the actual pull is performed only if the image changes. Because the daemon cache is cleared after each run, docker pull would not achieve the same.

RUN docker run hello-world executes the docker run command in the context of the daemon created by WITH DOCKER.

Loading images built by Earthly

A typical use of Docker in Earthly is running an image that has been built via Earthly itself. To achieve that, the option WITH DOCKER --load ...=... can be used. Here is an example:

SAVE IMAGE my-image:latest
FROM earthly/dind:alpine
WITH DOCKER --load test:latest=+build
RUN docker run test:latest

--load test:latest=+build takes the image produced by the target +build and loads it into the Docker daemon created by WITH DOCKER as the image with the tag test:latest. The tag can then be used to reference this image in other docker commands, such as docker run.

Notice that the image name produced as output is my-image:latest. This image name is not available in the WITH DOCKER environment, however, as it is only used to tag for use outside of Earthly. The name test:latest is used instead.

Running docker-compose

It is possible to run docker-compose via WITH DOCKER, either explicitly, simply by running the docker-compose tool, or implicitly, via the --compose flag. The --compose flag allows you to specify a Docker compose stack that needs to be brough up before the execution of the RUN command. For example:

FROM earthly/dind:alpine
--compose docker-compose.yml \
--service db \
--service api
RUN docker run some-integration-test:latest

Using the --compose flag has the added benefit that any images needed by the compose stack will be automatically added to the pull list by Earthly, thus using cache efficiently.

Integration testing

For more information on integration testing and working with service dependencies see our tutorial on integration testing in Earthly.

Limitations of Docker in Earthly

The current implementation of Docker in Earthly has a number of limitations:

  • Only one RUN command is allowed within the WITH DOCKER clause. The reason for this is that only one cache layer is used for the entire clause. You can, however, chain multiple shell commands together within a single RUN command. For example:

    RUN command1 && \
    command2 && \
    command3 && \
  • It is recommended that the target containing the WITH DOCKER clause inherits from a supported Docker-in-Docker (dind) image such as earthly/dind:alpine or earthly/dind:ubuntu. If your build requires the use of an alternative environment as part of a test (e.g. to run commands like sbt test or go test together with a docker-compose stack), consider placing the test itself in a Docker image, then loading that image via --load and running the test as a Docker container.

  • If you do not use an officially supported Docker-in-Docker image, Earthly will attempt to install Docker in whatever image you have chosen. This has the drawback of not being able to use cache efficiently and is not recommended for performance reasons.

  • To maximize the use of cache, all external images used should be declared via the options --pull or --compose. Even though commands such as docker run automatically pull an image if it is not found locally, it will do so every single time the WITH DOCKER clause is executed, due to Docker caching not being preserved between runs. Pre-declaring the images ensures that they are properly cached by Earthly to minimize unnecessary redownloads.

  • docker build cannot be used to build Dockerfiles. However, the Earthly command FROM DOCKERFILE can be used instead. See alternative to docker build below.

  • The state of the Docker daemon within Earthly cannot be inspected on the host (e.g. for debugging purposes). For example, if a docker-compose stack fails, you cannot execute commands like docker-compose logs or docker logs on the host. However, you may use the interactive mode to drop into a shell within the build environment and execute such commands there. For more information, see the debugging guide.

  • It is currently not possible to mount /var/run/docker.sock in order to use the host Docker daemon. This goes against Earthly's principles of keeping execution reproducible. Mounting the Docker socket may cause builds to depend on the host Daemon state (e.g. pre-cached images) in ways that may not be obvious or easy to reproduce if the build were executed in another environment.

  • Only one WITH DOCKER ... END clause may run at a time on a given host. If multiple such clauses are used in a build, Earthly will run them one by one sequentially. This is a temporary limitation and may be lifted in future versions of Earthly.

Alternatives to Docker in Earthly

It is not always necessary to execute docker commands within an Earthly build. Certain operations can be replicated with Earthly constructs.

Alternative to docker run

In certain cases, simple docker run invocations can be replaced by a simple RUN --entrypoint. For example, the following:

FROM docker:19.03.13-dind
WITH DOCKER --pull hello-world
RUN docker run hello-world

Can be rewritten as

FROM hello-world
RUN --entrypoint

This, of course, has limitations, such as not being able to mount volumes the same way docker run -v ... could (instead, a COPY command could be used); or not being able to run multiple containers in parallel. However, when appropriate, it can simplify a build definition.

Alternative to docker build

Running docker build within Earthly is discouraged, as it has a number of key limitations:

  • Layer caching does not work. This is because WITH DOCKER does not preserve Docker cache between runs (other than --pull).

  • Once an image is created, it cannot be exported as a build output in a form other than a TAR archive (e.g. it cannot be automatically loaded onto the host Docker daemon).

Instead of executing docker build, it is advisable to use the Earthly command FROM DOCKERFILE. For example, the command docker build -t my-image:latest . can be emulated by:

SAVE IMAGE my-image:latest

See also