Passing An Argument To Docker Build Step In Azure Pipeline

Last year, in this article, I talked about splitting the build and push steps of a Docker image.

Today, I’ll provide an example of how I passed the version computed from an Azure DevOps variable into the application.

Starting Point

In the previous article mentioned above, we learn how to split the build from the push step when using Docker in Azure.

Doing so allows to use to pass on arguments when build the Docker image. This can help perform many tasks, depending on what your needs are. But in this article, I’ll share one.

In my application, I have this module with a GET /version’ endpoint:

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import os

from flask import Blueprint, jsonify

version = Blueprint("version", __name__)

def get_version_from_file():
    version_file = os.path.join(os.path.dirname(__file__), 'version.txt')
    with open(version_file, 'r') as file:
        return file.read().strip()

@version.route("/", methods=["GET"])
def get_version():
    response = {
        "version": get_version_from_file()
    }
    return jsonify(response)

Given a text file version.txt placed in the module’s folder, querying GET /version would output the value stored into the text file.

Now, what are the steps to update so that the text file is filled with the new application version on each build?

Modifying the Pipelines

Evaluate the application version

That step works already. I invited to read the ”Evaluate the Image Tag” paragraph in the previous article.

We named the version evaluation step to setVersionStep and the actual version variable to fullVersion.

Pass on the version to Docker Build Step

Since we split the build and push steps, we can use the arguments property on the build Docker command:

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- task: Docker@2
  displayName: Build image
  inputs:
    command: build
    repository: $(imageRepository)
    dockerfile: $(dockerfilePath)
    buildContext: $(projectPath)
    containerRegistry: $(dockerRegistryServiceConnection)
    tags: |
      $(imageTags)
    # The only change from the previous article.
    arguments: --build-arg VERSION=$(setVersionStep.fullVersion)

Modify the Docker file

In the final step, we need to use the VERSION argument above in the Dockerfile.

First, let’s declare the argument:

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ARG VERSION

Then, let’s print the value as a debug trace:

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RUN echo "Version is <$VERSION>"

And finally, let’s write the version to that text file that the GET /version reads:

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RUN echo $VERSION > /project-container/app/modules/version/version.txt

Conclusion

That’s it! Now, you can pass on as many arguments as you need for your business logic.

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