# build service a
...

FROM base as service_b
# build service b
...
```

Compose file:

```yaml
services:
  a:
     build:
       target: service_a
  b:
     build:
       target: service_b
```

## Use another service's image as the base image

A popular pattern is to reuse a service image as a base image in another service.
As Compose does not parse the Dockerfile, it can't automatically detect this dependency 
between services to correctly order the build execution.

a.Dockerfile:

```dockerfile
FROM alpine
RUN /bin/sh -c apk add --update --no-cache openssl
```

b.Dockerfile:

```dockerfile
FROM service_a
# build service b
```

Compose file:

```yaml
services:
  a:
     image: service_a 
     build:
       dockerfile: a.Dockerfile
  b:
     image: service_b
     build:
       dockerfile: b.Dockerfile
```

Legacy Docker Compose v1 used to build images sequentially, which made this pattern usable
out of the box. Compose v2 uses BuildKit to optimise builds and build images in parallel 
and requires an explicit declaration.

The recommended approach is to declare the dependent base image as an additional build context:

Compose file:

```yaml
services:
  a:
     image: service_a
     build: 
       dockerfile: a.Dockerfile
  b:
     image: service_b
     build:
       dockerfile: b.Dockerfile
       additional_contexts:
         # `FROM service_a` will be resolved as a dependency on service "a" which has to be built first
         service_a: "service:a"
```

With the `additional_contexts` attribute, you can refer to an image built by another service without needing to explicitly name it:

b.Dockerfile:

```dockerfile

FROM base_image  
# `base_image` doesn't resolve to an actual image. This is used to point to a named additional context

# build service b
```

Compose file:

```yaml
services:
  a:
     build: 
       dockerfile: a.Dockerfile
       # built image will be tagged <project_name>_a
  b:
     build:
       dockerfile: b.Dockerfile
       additional_contexts:
         # `FROM base_image` will be resolved as a dependency on service "a" which has to be built first
         base_image: "service:a"
```

## Build with Bake

Using [Bake](/manuals/build/bake/_index.md) let you pass the complete build definition for all services
and to orchestrate build execution in the most efficient way. 

To enable this feature, run Compose with the `COMPOSE_BAKE=true` variable set in your environment.

```console
$ COMPOSE_BAKE=true docker compose build
[+] Building 0.0s (0/1)                                                         
 => [internal] load local bake definitions                                 0.0s
...
[+] Building 2/2 manifest list sha256:4bd2e88a262a02ddef525c381a5bdb08c83  0.0s
 ✔ service_b  Built                                                        0.7s 
 ✔ service_a  Built    
```

Bake can also be selected as the default builder by editing your `$HOME/.docker/config.json` config file:
```json
{
  ...
  "plugins": {
    "compose": {
      "build": "bake"
    }
  }
  ...
}
```