Allow `x-*` extension on 3rd level objects

As for top-level key, any 3rd-level key which starts with `x-` will be
ignored by compose. This allows for users to:
* include additional metadata in their compose files
* create YAML anchor objects that can be re-used in other parts of the config

This matches a similar feature in the swagger spec definition:
https://swagger.io/specification/#specificationExtensions

This means a composefile like the following is valid

```
verison: "3.7"
services:
  foo:
    image: foo/bar
    x-foo: bar
network:
  bar:
    x-bar: baz
```

It concerns services, volumes, networks, configs and secrets.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

cli/compose/schema/bindata.go

@@ -467,44 +467,44 @@ DoTuq9lAU9Q4O1xV/59X/w8AAP//zRo7vm9CAAA=
 
 	"/data/config_schema_v3.7.json": {
 		local:   "data/config_schema_v3.7.json",
-		size:    17540,
+		size:    17740,
 		modtime: 1518458244,
 		compressed: `
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 `,
 	},
 

cli/compose/schema/data/config_schema_v3.7.json

@@ -319,6 +319,7 @@
         },
         "working_dir": {"type": "string"}
       },
+      "patternProperties": {"^x-": {}},
       "additionalProperties": false
     },
 
@@ -489,6 +490,7 @@
         "attachable": {"type": "boolean"},
         "labels": {"$ref": "#/definitions/list_or_dict"}
       },
+      "patternProperties": {"^x-": {}},
       "additionalProperties": false
     },
 
@@ -513,6 +515,7 @@
         },
         "labels": {"$ref": "#/definitions/list_or_dict"}
       },
+      "patternProperties": {"^x-": {}},
       "additionalProperties": false
     },
 
@@ -530,6 +533,7 @@
         },
         "labels": {"$ref": "#/definitions/list_or_dict"}
       },
+      "patternProperties": {"^x-": {}},
       "additionalProperties": false
     },
 
@@ -547,6 +551,7 @@
         },
         "labels": {"$ref": "#/definitions/list_or_dict"}
       },
+      "patternProperties": {"^x-": {}},
       "additionalProperties": false
     },
 

cli/compose/schema/schema_test.go

@@ -45,6 +45,39 @@ func TestValidateAllowsXTopLevelFields(t *testing.T) {
 	assert.NilError(t, err)
 }
 
+func TestValidateAllowsXFields(t *testing.T) {
+	config := dict{
+		"version": "3.7",
+		"services": dict{
+			"bar": dict{
+				"x-extra-stuff": dict{},
+			},
+		},
+		"volumes": dict{
+			"bar": dict{
+				"x-extra-stuff": dict{},
+			},
+		},
+		"networks": dict{
+			"bar": dict{
+				"x-extra-stuff": dict{},
+			},
+		},
+		"configs": dict{
+			"bar": dict{
+				"x-extra-stuff": dict{},
+			},
+		},
+		"secrets": dict{
+			"bar": dict{
+				"x-extra-stuff": dict{},
+			},
+		},
+	}
+	err := Validate(config, "3.7")
+	assert.NilError(t, err)
+}
+
 func TestValidateSecretConfigNames(t *testing.T) {
 	config := dict{
 		"version": "3.5",