support additional_context reference to another service

[ndeloof]

• classic builder doesn't support additional context
• buildkit builder detects service: prefix in contexts and builds images in order (in addition to runtime dependencies, to preserve backward compatibility)
• bake builder delegates to buildx bake with a named context for service's target

$ COMPOSE_BAKE=false docker compose build
INFO[0000] additional_context with "service:"" is better supported when delegating build go bake. Set COMPOSE_BAKE=true 
[+] Building 2.0s (16/16) FINISHED                                                                                                       docker:desktop-linux
 => [a internal] load build definition from Dockerfile.a                                                                                                 0.0s
...
 => => naming to docker.io/library/b:latest                                                                                                              0.0s
 => => unpacking to docker.io/library/b:latest                                                                                                           0.0s
 => [b] resolving provenance for metadata file                                                                                                           0.0s
[+] Building 2/2
 ✔ a  Built                                                                                                                                              0.0s 
 ✔ b  Built