bake: support compose additional contexts

compose-go v1.13.0 supports the new additional_contexts to allow passing
additional build context during build, so we should map this to bake's
contexts property.

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/compose.go

@@ -75,6 +75,14 @@ func ParseCompose(cfgs []compose.ConfigFile, envs map[string]string) (*Config, e
 				dockerfileInlineP = &dockerfileInline
 			}
 
+			var additionalContexts map[string]string
+			if s.Build.AdditionalContexts != nil {
+				additionalContexts = map[string]string{}
+				for k, v := range s.Build.AdditionalContexts {
+					additionalContexts[k] = v
+				}
+			}
+
 			var secrets []string
 			for _, bs := range s.Build.Secrets {
 				secret, err := composeToBuildkitSecret(bs, cfg.Secrets[bs.Source])
@@ -95,6 +103,7 @@ func ParseCompose(cfgs []compose.ConfigFile, envs map[string]string) (*Config, e
 			t := &Target{
 				Name:             targetName,
 				Context:          contextPathP,
+				Contexts:         additionalContexts,
 				Dockerfile:       dockerfilePathP,
 				DockerfileInline: dockerfileInlineP,
 				Tags:             s.Build.Tags,

bake/compose_test.go

@@ -21,6 +21,8 @@ services:
   webapp:
     build:
       context: ./dir
+      additional_contexts:
+        foo: /bar
       dockerfile: Dockerfile-alternate
       network:
         none
@@ -63,6 +65,7 @@ secrets:
 
 	require.Equal(t, "webapp", c.Targets[1].Name)
 	require.Equal(t, "./dir", *c.Targets[1].Context)
+	require.Equal(t, map[string]string{"foo": "/bar"}, c.Targets[1].Contexts)
 	require.Equal(t, "Dockerfile-alternate", *c.Targets[1].Dockerfile)
 	require.Equal(t, 1, len(c.Targets[1].Args))
 	require.Equal(t, ptrstr("123"), c.Targets[1].Args["buildno"])