# Build a Docker image

{% hint style="info" %}
Avoid using capital letters when referencing a DuploCloud construct, such as a Tenant, even when the UI displays the string in all capital letters. Don't specify DEV01, for example; specify **dev01**. The \`TENANT\_NAME\` may need to be lowercase even though the UI shows it in uppercase.
{% endhint %}

## Build and Push to ECR

This section aims to show you how to build a Docker image and push it to ECR.

It does three things:

* Logs in to AWS ECR (using just-in-time AWS credentials from DuploCloud)
* Builds and tags your Docker image, with the tags based on the Git commit SHA and ref.
* Pushes your Docker image

### Example Workflow

Here is an example of a GitHub workflow that builds and pushes a Docker image to ECR.

To use it, ensure the following are configured correctly:

* `DUPLO_HOST` environment variable
* `DUPLO_TOKEN` environment variable

<pre class="language-yaml"><code class="lang-yaml">name: Publish Image
on:
  # Triggers the workflow on push to matching branches
  push:
    branches:
    - main
  # (Optional) Allows users to trigger the workflow manually from the GitHub UI
  workflow_dispatch: {}
  # (Optional) Allow other workflows to use this workflow and its outputs
  workflow_call: 
    outputs:
      image:
        description: The URI of the image
        value: ${{ jobs.build_image.outputs.image }}
    secrets:
      DUPLO_TOKEN:
        description: The token to use for DuploCloud API calls
        required: true

env:
  DUPLO_HOST: ${{ vars.DUPLO_HOST }}
  DUPLO_TOKEN: ${{ secrets.DUPLO_TOKEN }}
  # Images are usually stored in a dedicated tenant, so the name doesn't change
  DUPLO_TENANT: devops

jobs:
  build_image:
    name: Build and Push Image
    runs-on: ubuntu-latest
    outputs:
      image: ${{ steps.build_image.outputs.uri }}
    steps:

    - name: Checkout code
      uses: actions/checkout@v4

    # Configures DuploCloud and the host cloud, for example, AWS
    - name: Cloud CI Setup
      uses: <a data-footnote-ref href="#user-content-fn-1">duplocloud/actions@v0.0.12</a>
    
    # logs into the registry, configures Docker, builds the image, and lastly pushes
    - name: Build and Push Docker Image
      id: build_image
      uses: <a data-footnote-ref href="#user-content-fn-2">duplocloud/actions/build-image@</a>v0.0.5
      with:
        platforms: linux/amd64,linux/arm64
        push: true # false for dry runs
        build-args: >
          foo=bar
          ice_cream=chocolate
          name=${{ github.repository }}
</code></pre>

### References

* [DuploCloud Build Image Action for GitHub](https://github.com/duplocloud/actions/tree/main/build-image)
* [NPM Pipeline Example](https://github.com/duplocloud/npm-pipeline-example/tree/main/.github/workflows)
* [GitHub on publishing Docker images](https://docs.github.com/en/actions/publishing-packages/publishing-docker-images)
* [Docker introduction to GitHub Actions](https://docs.docker.com/build/ci/github-actions/)

[^1]: <https://github.com/duplocloud/actions>

[^2]: <https://github.com/duplocloud/actions/tree/main/build-image>


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.duplocloud.com/docs/automation-platform/introduction-to-ci-cd/github-actions/build-a-docker-image.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.