Add features.md to formalize the runc features JSON

Add `features.md` and `features-linux.md`, to formalize the `runc features` JSON that was introduced in runc v1.1.0.

A runtime caller MAY use this JSON to detect the features implemented by the runtime.

The spec corresponds to https://github.com/opencontainers/runc/blob/v1.1.0/types/features/features.go
(opencontainers/runc PR 3296, opencontainers/runc PR 3310)

Differences since runc v1.1.0:
- Add `.linux.intelRdt.enabled` field
- Add `.linux.cgroup.rdma` field
- Add `.linux.seccomp.knownFlags` and `.linux.seccomp.supportedFlags` fields (Implemented in runc PR 3588)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

schema/features-linux.json

@@ -0,0 +1,103 @@
+{
+    "linux": {
+        "description": "Linux platform-specific features",
+        "type": "object",
+        "properties": {
+            "namespaces": {
+                "type": "array",
+                "items": {
+                    "$ref": "defs-linux.json#/definitions/NamespaceType"
+                }
+            },
+            "capabilities": {
+                "type": "array",
+                "items": {
+                    "type": "string",
+                    "pattern": "^CAP_[A-Z_]+$"
+                }
+            },
+            "cgroup": {
+                "type": "object",
+                "properties": {
+                    "v1": {
+                        "type": "boolean"
+                    },
+                    "v2": {
+                        "type": "boolean"
+                    },
+                    "systemd": {
+                        "type": "boolean"
+                    },
+                    "systemdUser": {
+                        "type": "boolean"
+                    },
+                    "rdma": {
+                        "type": "boolean"
+                    }
+                }
+            },
+            "seccomp": {
+                "type": "object",
+                "properties": {
+                    "enabled": {
+                        "type": "boolean"
+                    },
+                    "actions": {
+                        "type": "array",
+                        "items": {
+                            "$ref": "defs-linux.json#/definitions/SeccompAction"
+                        }
+                    },
+                    "operators": {
+                        "type": "array",
+                        "items": {
+                            "$ref": "defs-linux.json#/definitions/SeccompOperators"
+                        }
+                    },
+                    "archs": {
+                        "type": "array",
+                        "items": {
+                            "$ref": "defs-linux.json#/definitions/SeccompArch"
+                        }
+                    },
+                    "knownFlags": {
+                        "type": "array",
+                        "items": {
+                            "$ref": "defs-linux.json#/definitions/SeccompFlag"
+                        }
+                    },
+                    "supportedFlags": {
+                        "type": "array",
+                        "items": {
+                            "$ref": "defs-linux.json#/definitions/SeccompFlag"
+                        }
+                    }
+                }
+            },
+            "apparmor": {
+                "type": "object",
+                "properties": {
+                    "enabled": {
+                        "type": "boolean"
+                    }
+                }
+            },
+            "selinux": {
+                "type": "object",
+                "properties": {
+                    "enabled": {
+                        "type": "boolean"
+                    }
+                }
+            },
+            "intelRdt": {
+                "type": "object",
+                "properties": {
+                    "enabled": {
+                        "type": "boolean"
+                    }
+                }
+            }
+        }
+    }
+}