Build Arguments

4 minute read

This article is about using Docker build arguments with CloudBees CodeShip Pro. If you are unfamiliar with build arguments, we recommend reading Docker’s build arguments documentation. If you are unfamiliar with CloudBees CodeShip Pro, we recommend our getting started guide or the features overview page.

Note that you will also need to use the CodeShip Pro local CLI tool to encrypt your build arguments.

Overview: Build Arguments

For each service, you can declare build arguments, which are values available to the image only at build time. For example, if you must pass the image a set of credentials in order to access an asset or repository when the image is built, you would pass that value to the image as a build argument.

Build Arguments vs. Environment Variables

During a build on CodeShip’s Docker platform, there are three ways to pass custom values to your services:

  • Build arguments or encrypted build arguments: available only at image build time

  • ENV variable declared in Dockerfile: available at build time and run time

  • environment or encrypted environment: available only at run time

Build arguments are unique in that they are available only at build time. They are not persisted in the image. However, you may assign a build argument to an environment variable during the build process, and that environment variable will be available.

Unencrypted Build Arguments

Declaring build arguments in your services file requires updates in two places: the service’s Dockerfile and the codeship-services.yml file.

Dockerfile ARG instruction

The service’s Dockerfile must include the ARG instruction, which declares the name of the argument you will pass at build time. You may also declare a default here.

FROM ubuntu:latest ARG build_env # build_env=test would make test the default value RUN "$build_env"

Passing build args in the services file

Now that the Dockerfile knows to expect an argument, you can pass the argument to the image via the service configuration in codeship-services.yml.

app: build: dockerfile: Dockerfile args: build_env: staging

When the app service is built, the value of build_env becomes staging. If no value was set, build_env would remain test. You are not required to declare a default, but you must add an ARG instruction to the Dockerfile for each build argument you pass in via codeship-services.yml.

You may also declare a build argument before the FROM instruction, which is a new feature introduced in Docker 17.05. Following the same pattern, you must first declare the argument before consuming it.


Note: YAML boolean values (true, false, yes, no, on, off) must be enclosed in quotes, so that the parser interprets them as strings.

Encrypted Build Arguments

In a lot of cases, the values needed by the image at build time are secrets – credentials, passwords, and other things that you don’t want to check in to source control in plain text.

Because of this, CodeShip supports encrypted build arguments. You can either encrypt a build argument individually, or encrypt an entire file containing all of the build arguments you need.

First, create a file in the root directory - in this case, a file named build_args. You will also need to download the project AES key to the root directory (and add it to the .gitignore file).

If you need to reset your AES key you can do so by visiting Project Settings > General and clicking Reset project AES key.


Take care to use KEY=value syntax and not key: value.

Once the AES key is in the root directory, run the jet encrypt command with an input and an output filename: jet encrypt build_args build_args.encrypted (Learn more about using Jet)

Pass that file to your service’s build directive.

app: build: dockerfile: Dockerfile encrypted_args_file: build_args.encrypted

If your use case is simple enough, you may want to pass in encrypted values directly instead of requiring CodeShip to read them from a file. The following syntax is also supported:

app: build: dockerfile: Dockerfile encrypted_args: 8rD1P1xO1CwB4L99JBqnvoSOX+1wimf9qwHXATf9foasPtU6Sw==

CodeShip will decrypt your build arguments and pass them to the image when it is built.

CI/CD Variables As Build Arguments

CodeShip sets a variety of CI/CD-related environment variables at run time with information about your build.

These can be set as build arguments and used in the Dockerfile, but they must be explicitly set as unencrypted arguments in your codeship-services.yml file.

Note that there are several key things to know when using these values as build arguments:

  • Not all of these values will be available locally via jet since they may depend on information from your SCM.

  • These arguments will still need to be declared in your Dockerfile similar to any other build argument.

  • These values use a slightly different format than you may be used to, as they are set by the Golang template library just like image push steps use.

Here’s an example of declaring a default value as a build argument:

app: build: dockerfile: Dockerfile args: BRANCH: "{{ .Branch }}" CI: "{{ .Ci }}"

The full list of CI/CD-related variables is:

  • ProjectID (the CodeShip defined project ID)

  • BuildID (the CodeShip defined build ID)

  • RepoName (the name of the repository according to the SCM)

  • Branch (the name of the current branch)

  • CommitID (the commit hash or ID)

  • CommitMessage (the commit message)

  • CommitDescription (the commit description, see footnote)

  • CommitterName (the name of the person who committed the change)

  • CommitterEmail (the email of the person who committed the change)

  • CommitterUsername (the username of the person who committed the change)

  • Time (a golang Time object of the build time)

  • Timestamp (a unix timestamp of the build time)

  • StringTime (a readable version of the build time)

  • Ci (defaults to true)

Impact on Docker Image Caching

Docker will attempt to reuse layers of your image if there are no changes. Although the values of a build argument are not persisted to the image, they do impact the build cache in a similar way. Refer to Docker’s notes on cache invalidation when using build arguments.

Common Error Messages

One or more build-args [XXX] were not consumed

If you pass a build argument to an image at build time, but do not have a corresponding ARG instruction in the Dockerfile, Docker (versions < 1.13) will fail the image build.

As of 1.13, unused build args will not fail the build, but will generate a warning message.