Fix build section without context.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/config/config.py

@@ -878,6 +878,9 @@ def validate_paths(service_dict):
             build_path = build
         elif isinstance(build, dict) and 'context' in build:
             build_path = build['context']
+        else:
+            # We have a build section but no context, so nothing to validate
+            return
 
         if (
             not is_url(build_path) and

compose/config/service_schema_v2.0.json

@@ -195,7 +195,12 @@
       "anyOf": [
           {"required": ["build"]},
           {"required": ["image"]}
-      ]
+      ],
+      "properties": {
+        "build": {
+          "required": ["context"]
+        }
+      }
     }
   }
 }

compose/config/validation.py

@@ -253,10 +253,9 @@ def handle_generic_service_error(error, path):
         msg_format = "{path} contains an invalid type, it should be {msg}"
         error_msg = _parse_valid_types_from_validator(error.validator_value)
 
-    # TODO: no test case for this branch, there are no config options
-    # which exercise this branch
     elif error.validator == 'required':
-        msg_format = "{path} is invalid, {msg}"
+        error_msg = ", ".join(error.validator_value)
+        msg_format = "{path} is invalid, {msg} is required."
 
     elif error.validator == 'dependencies':
         config_key = list(error.validator_value.keys())[0]