NpmAuthenticate in Docker context on Azure DevOps pipeline

Are you utilizing a Docker container for your application builds? If so, and you’re implementing a staged build strategy, you might need to authenticate with your Azure DevOps artifact repository from within the build container. Here’s how to achieve that:

1. Create a copy of .npmrc file for it to be enriched with credentials

    # a bash task in your Azure pipeline
    cp -a src/.npmrc src/.user_npmrc
   

2. Use the NpmAuthenticate task in your pipeline to generate the credentials for the private registry. Note: Be sure to specify .user_npmrc as workingFile. We’ll see later why. Also, being a copy of .npmrc it specifies the registries you want to work with. For example, for a file that lives under a src subdir in the project root:

    steps:
    - task: npmAuthenticate@0
      inputs:
        workingFile: $(System.DefaultWorkingDirectory)/src/.user_npmrc
   

3. Add a COPY directive to your Dockerfile to copy .user_npmrc , now with credentials, inside the image and be used by npm.

   Tip: Copy it to /tmp to prevent private registry credentials to be eventually included in the finalized Docker image.

    COPY .user_npmrc /tmp/.npmrc
   

4. Modify your Dockerfile to specify the path to the npm user config using NPM_CONFIG_USERCONFIG environment variable in an ARG statement.

   Note: ARG represents environment variables utilized only during Docker image build time. This step is needed because, by default, npm checks for credentials in the (other) .npmrc file, the one in the user’s home folder, but this time we want it to use the one from /tmp we copied in the previous step.

    ARG NPM_CONFIG_USERCONFIG=/tmp/.npmrc
   

Important: Ensure these lines are placed above the npm install command in your Dockerfile! By doing so, the path to your user .npmrc file will be accessible during your npm install command allowing node modules in private registries to be downloaded and installed.

In the end, your Dockerfile should look like this:

FROM node:18
# ...
ARG NPM_CONFIG_USERCONFIG=/tmp/.npmrc
COPY .user_npmrc /tmp/.npmrc
COPY package.json package-lock.json .npmrc ./
COPY src ./src
RUN npm install --omit=dev
# ...

That’s it!

I hope it helps. Thanks for reading.