buildx/docs/guides/bake/build-contexts.md

title	keywords
Defining additional build contexts and linking targets	build, buildx, bake, buildkit, hcl

In addition to the main context key that defines the build context each target can also define additional named contexts with a map defined with key contexts. These values map to the --build-context flag in the build command.

Inside the Dockerfile these contexts can be used with the FROM instruction or --from flag.

The value can be a local source directory, container image (with docker-image:// prefix), Git URL, HTTP URL or a name of another target in the Bake file (with target: prefix).

Pinning alpine image

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

# docker-bake.hcl
target "app" {
  contexts = {
    alpine = "docker-image://alpine:3.13"
  }
}

Using a secondary source directory

# syntax=docker/dockerfile:1
FROM scratch AS src

FROM golang
COPY --from=src . .

# docker-bake.hcl
target "app" {
  contexts = {
    src = "../path/to/source"
  }
}

Using a result of one target as a base image in another target

To use a result of one target as a build context of another, specity the target name with target: prefix.

# syntax=docker/dockerfile:1
FROM baseapp
RUN echo "Hello world"

# docker-bake.hcl
target "base" {
  dockerfile = "baseapp.Dockerfile"
}

target "app" {
  contexts = {
    baseapp = "target:base"
  }
}

Please note that in most cases you should just use a single multi-stage Dockerfile with multiple targets for similar behavior. This case is recommended when you have multiple Dockerfiles that can't be easily merged into one.