Defining additional build contexts and linking targets

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.

Supported context values are:

• Local filesystem directories
• Container images
• Git URLs
• HTTP URLs
• Name of another target in the Bake file

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"
  }
}

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