Remove duplicate values check for build.cache_from

The `docker` command accepts duplicate values, so there is no benefit to
performing this check.

Fixes #7342.

Signed-off-by: Mark Gallagher <[email protected]>

compose/config/compose_spec.json

@@ -87,7 +87,7 @@
                 "dockerfile": {"type": "string"},
                 "args": {"$ref": "#/definitions/list_or_dict"},
                 "labels": {"$ref": "#/definitions/list_or_dict"},
-                "cache_from": {"$ref": "#/definitions/list_of_strings"},
+                "cache_from": {"type": "array", "items": {"type": "string"}},
                 "network": {"type": "string"},
                 "target": {"type": "string"},
                 "shm_size": {"type": ["integer", "string"]},

tests/unit/config/config_test.py

@@ -669,7 +669,7 @@ class ConfigTest(unittest.TestCase):
 
             assert 'Invalid service name \'mong\\o\'' in excinfo.exconly()
 
-    def test_config_duplicate_cache_from_values_validation_error(self):
+    def test_config_duplicate_cache_from_values_no_validation_error(self):
         with pytest.raises(ConfigurationError) as exc:
             config.load(
                 build_config_details({
@@ -681,7 +681,7 @@ class ConfigTest(unittest.TestCase):
                 })
             )
 
-        assert 'build.cache_from contains non-unique items' in exc.exconly()
+        assert 'build.cache_from contains non-unique items' not in exc.exconly()
 
     def test_load_with_multiple_files_v1(self):
         base_file = config.ConfigFile(