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](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-context).
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
```dockerfile
# syntax=docker/dockerfile:1
FROM alpine
RUN echo "Hello world"
```
```hcl
# docker-bake.hcl
target "app" {
contexts = {
alpine = "docker-image://alpine:3.13"
}
}
```
## Using a secondary source directory
```dockerfile
# syntax=docker/dockerfile:1
FROM scratch AS src
FROM golang
COPY --from=src . .
```
```hcl
# 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.
```dockerfile
# syntax=docker/dockerfile:1
FROM baseapp
RUN echo "Hello world"
```
```hcl
# 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.
