named-contexts.md

Named contexts

You can define additional build contexts that can be accessed in your Dockerfile with FROM name or --from=name. When Dockerfile defines a stage with the same name it is overwritten.

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

Pin image to a specific 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@v3
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v2
      -
        name: Build
        uses: docker/build-push-action@v3
        with:
          context: .
          build-contexts: |
            alpine=docker-image://alpine:3.16
          tags: myimage:latest

Usage of the built image in other build steps

By default, the setup-buildx action uses docker-container as a build driver, so built Docker images are not available in the builder container.

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@v3
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v2
      -
        name: Build base image
        uses: docker/build-push-action@v3
        with:
          context: base
          load: true
          tags: my-base-image:latest
      -
        name: Build
        uses: docker/build-push-action@v3
        with:
          context: .
          build-contexts: |
            alpine=docker-image://my-base-image:latest
          tags: myimage:latest