Add build constraints to .goxc.json

Author: andystanton

.goxc.json

@@ -1,13 +1,14 @@
 {
-    "PackageVersion": "1.0.4",
-    "ConfigVersion": "0.9",
-    "TaskSettings": {
-        "bintray": {
-            "downloadspage": "bintray.md",
-            "repository": "release",
-            "package": "dexec",
-            "user": "docker-exec-bot",
-            "subject": "dexec"
-        }
-    }
+    "BuildConstraints": "windows,linux,darwin,openbsd",
+	"PackageVersion": "1.0.4",
+	"TaskSettings": {
+		"bintray": {
+			"downloadspage": "bintray.md",
+			"package": "dexec",
+			"repository": "release",
+			"subject": "dexec",
+			"user": "docker-exec-bot"
+		}
+	},
+	"ConfigVersion": "0.9"
 }