Add support of tags in the build section

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

build.md

@@ -382,6 +382,19 @@ Service builds MAY be granted access to multiple secrets. Long and short syntax
 same Compose file. Defining a secret in the top-level `secrets` MUST NOT imply granting any service build access to it.
 Such grant must be explicit within service specification as [secrets](spec.md#secrets) service element.
 
+### tags
+
+`tags` defines a list of tag mappings that MUST be associated to the build image.
+
+```yml
+tags:
+  "myimage:mytag"
+  "registry/username/myrepos:my-other-tag"
+```
+
+Compose implementations MUST create matching entry with the IP address and hostname in the container's network
+configuration, which means for Linux `/etc/hosts` will get extra lines:
+
 ## Implementations
 
 * [docker-compose](https://docs.docker.com/compose)

schema/compose-spec.json

@@ -102,7 +102,8 @@
                 "shm_size": {"type": ["integer", "string"]},
                 "extra_hosts": {"$ref": "#/definitions/list_or_dict"},
                 "isolation": {"type": "string"},
-                "secrets": {"$ref": "#/definitions/service_config_or_secret"}
+                "secrets": {"$ref": "#/definitions/service_config_or_secret"},
+                "tags":{"type": "array", "items": {"type": "string"}}
               },
               "additionalProperties": false,
               "patternProperties": {"^x-": {}}