Named contexts with GitHub Actions

Table of contents

You can define additional build contexts, and access them in your Dockerfile with FROM name or --from=name. When Dockerfile defines a stage with the same name it's overwritten.

This can be useful with GitHub Actions to reuse results from other builds or pin an image to a specific tag in your workflow.

Pin image to a tag

Replace alpine:latest with a pinned one:

# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello World"

name: ci

on:
  push:
    branches:
      - "main"

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
      - name: Build
        uses: docker/build-push-action@v5
        with:
          context: .
          build-contexts: |
            alpine=docker-image://alpine:3.19            
          tags: myimage:latest

Use image in subsequent steps

By default, the Docker Setup Buildx action uses docker-container as a build driver, so built Docker images aren't loaded automatically.

With named contexts you can reuse the built image:

# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello World"

name: ci

on:
  push:
    branches:
      - "main"

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
        with:
          driver: docker
      - name: Build base image
        uses: docker/build-push-action@v5
        with:
          context: ./base
          file: ./base/Dockerfile
          load: true
          tags: my-base-image:latest
      - name: Build
        uses: docker/build-push-action@v5
        with:
          context: .
          build-contexts: |
            alpine=docker-image://my-base-image:latest            
          tags: myimage:latest

Using with a container builder

As shown in the previous section we are not using the default docker-container driver for building with named contexts. That's because this driver can't load an image from the Docker store as it's isolated. To solve this problem you can use a local registry to push your base image in your workflow:

# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello World"

name: ci

on:
  push:
    branches:
      - "main"

jobs:
  docker:
    runs-on: ubuntu-latest
    services:
      registry:
        image: registry:2
        ports:
          - 5000:5000
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      - name: Set up QEMU
        uses: docker/setup-qemu-action@v3
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
        with:
          # network=host driver-opt needed to push to local registry
          driver-opts: network=host
      - name: Build base image
        uses: docker/build-push-action@v5
        with:
          context: ./base
          file: ./base/Dockerfile
          tags: localhost:5000/my-base-image:latest
          push: true
      - name: Build
        uses: docker/build-push-action@v5
        with:
          context: .
          build-contexts: |
            alpine=docker-image://localhost:5000/my-base-image:latest            
          tags: myimage:latest