Add support for `additional_contexts` in `build` service config

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

e2e/cucumber-features/build-contexts.feature

@@ -0,0 +1,23 @@
+Feature: Build Contexts
+
+Background:
+    Given a compose file
+        """
+        services:
+          a:
+            build:
+                context: .
+                additional_contexts:
+                  - dep=docker-image://ubuntu:latest
+        """
+    And a dockerfile
+        """
+        # syntax=docker/dockerfile:1
+        FROM alpine:latest
+        COPY --from=dep /etc/hostname /
+        """
+
+Scenario: Build w/ build context
+    When I run "compose build"
+    Then the exit code is 0
+

pkg/compose/build.go

@@ -80,6 +80,9 @@ func (s *composeService) build(ctx context.Context, project *types.Project, opti
 			if err != nil {
 				return err
 			}
+			if len(service.Build.AdditionalContexts) > 0 {
+				buildOptions.Inputs.NamedContexts = toBuildContexts(service.Build.AdditionalContexts)
+			}
 			for _, image := range service.Build.CacheFrom {
 				buildOptions.CacheFrom = append(buildOptions.CacheFrom, bclient.CacheOptionsEntry{
 					Type:  "registry",
@@ -200,7 +203,6 @@ func (s *composeService) getBuildOptions(project *types.Project, images map[stri
 		}
 	}
 	return opts, nil
-
 }
 
 func (s *composeService) getLocalImagesDigests(ctx context.Context, project *types.Project) (map[string]string, error) {
@@ -422,6 +424,14 @@ func getImageBuildLabels(project *types.Project, service types.ServiceConfig) ty
 	return ret
 }
 
+func toBuildContexts(additionalContexts map[string]*string) map[string]build.NamedContext {
+	namedContexts := map[string]build.NamedContext{}
+	for name, buildContext := range additionalContexts {
+		namedContexts[name] = build.NamedContext{Path: *buildContext}
+	}
+	return namedContexts
+}
+
 func useDockerDefaultPlatform(project *types.Project, platformList types.StringList) ([]specs.Platform, error) {
 	var plats []specs.Platform
 	if platform, ok := project.Environment["DOCKER_DEFAULT_PLATFORM"]; ok {

pkg/compose/build_classic.go

@@ -46,7 +46,7 @@ import (
 )
 
 func (s *composeService) doBuildClassic(ctx context.Context, project *types.Project, opts map[string]buildx.Options) (map[string]string, error) {
-	var nameDigests = make(map[string]string)
+	nameDigests := make(map[string]string)
 	var errs error
 	err := project.WithServices(nil, func(service types.ServiceConfig) error {
 		imageName := api.GetImageNameOrDefault(service, project.Name)
@@ -103,6 +103,9 @@ func (s *composeService) doBuildClassicSimpleImage(ctx context.Context, options
 	if utils.Contains(options.Allow, entitlements.EntitlementSecurityInsecure) {
 		return "", errors.Errorf("this builder doesn't support privileged mode, set DOCKER_BUILDKIT=1 to use builder supporting privileged mode")
 	}
+	if len(options.Inputs.NamedContexts) > 0 {
+		return "", errors.Errorf("this builder doesn't support additional contexts, set DOCKER_BUILDKIT=1 to use BuildKit which does")
+	}
 
 	if options.Labels == nil {
 		options.Labels = make(map[string]string)