docker-build-push/docs/advanced/named-contexts.md
CrazyMax 0f5a7d48d5
docs: named contexts
Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>
2022-10-07 18:22:40 +02:00

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# Named contexts
You can define [additional build contexts](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-context)
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:
```dockerfile
# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello World"
```
```yaml
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](https://github.com/docker/setup-buildx-action#about)
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:
```dockerfile
# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello World"
```
```yaml
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
```