schema: add tmpfs.mode field

Schema included `size` for `tmpfs` mounts but not `mode`. This
appears to have been an oversight.

Fixes #302.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

schema/compose-spec.json

@@ -406,6 +406,9 @@
                           {"type": "integer", "minimum": 0},
                           {"type": "string"}
                         ]
+                      },
+                      "mode": {
+                        "type": "number"
                       }
                     },
                     "additionalProperties": false,

types/types.go

@@ -828,6 +828,8 @@ type ServiceVolumeVolume struct {
 type ServiceVolumeTmpfs struct {
 	Size UnitBytes `yaml:",omitempty" json:"size,omitempty"`
 
+	Mode uint32 `yaml:",omitempty" json:"mode,omitempty"`
+
 	Extensions map[string]interface{} `yaml:",inline" json:"-"`
 }