Build Environments

Read the Docs uses container virtualization to encapsulate documentation build processes. Each build spins up a new virtual machine using our base image, which is an image with the minimum necessary components required to build documentation. Virtual machines are limiting in CPU time and memory, which aims to reduce excessive usage of build resources.


Build environments use Docker to handle container virtualization. To perform any development on the Docker build system, you will need to set up Docker on your host system. Setup of Docker will vary by system, and so is out of the scope of this documentation.

Once you have Docker set up, you will need to pull down our build image. These images are found on our Docker Hub repository, the source comes from our container image repo.


The size of the Docker images is around 5 to 9 GB.

To get started using Docker for build environments, you’ll need to pull down at least one build image. For example, to pull down our latest image:

docker pull readthedocs/build:latest

The default image used by our build servers is readthedocs/build:latest. This would be a good place to start testing as the latest version could operate differently. See DOCKER_IMAGE below for setting this configuration option.

After this image is downloaded, you can update your settings to use the new image – see Configuration.


There are several settings used to configure usage of virtual machines:


True/False value used to enable the Docker build environment.

Default: False


A dictionary of limits to virtual machines. These limits include:


An integer representing the total allowed time limit (in seconds) of build processes. This time limit affects the parent process to the virtual machine and will force a virtual machine to die if a build is still running after the allotted time expires.


The maximum memory allocated to the virtual machine. If this limit is hit, build processes will be automatically killed. Examples: ‘200m’ for 200MB of total memory, or ‘2g’ for 2GB of total memory.

Default: {'memory': '1g', 'time': 600}


Tag of a Docker image to use as a base image.

Default: readthedocs/build:latest


URI of the socket to connect to the Docker daemon. Examples include: unix:///var/run/docker.sock and tcp://

Default: unix:///var/run/docker.sock


Version of the API to use for the Docker API client.

Default: auto

Local development

On Linux development environments, it’s likely that your UID and GID do not match the docs user that is set up as the default user for builds. In this case, it’s necessary to make a new image that overrides the UID and GID for the normal container user:

contrib/ latest

This will create a new image, readthedocs/build-dev:latest. To build a different image, you can instead specify a version to build:

contrib/ 5.0

This will create a new image, readthedocs/build-dev:5.0.

You can set a option to automatically patch the image names to the development image names that are built here:


If set to True, replace the normal Docker image name used in building readthedocs/build with the image name output for these commands, readthedocs/build-dev.

Builder responsibility

Builders have a very specific job. They take the updated source code and generate the correct artifacts. The code lives in self.version.project.checkout_path(self.version.slug). The artifacts should end up in self.version.project.artifact_path(version=self.version.slug, type=self.type) Where type is the name of your builder. All files that end up in the artifact directory should be in their final form.

Writing your own builder


Builds happen on a server using only the RTD Public API. There is no reason that you couldn’t build your own independent builder that wrote into the RTD namespace, but that would require work on our side if you can convince us :)

The documentation build system in RTD is made pluggable, so that you can build out your own backend. If you have a documentation format that isn’t currently supported, you can add support by contributing a backend.

The builder backends detail the higher level parts of the API that you need to implement. A basic run goes something like this:

backend = get_backend(project.documentation_type)
if force:
if success: