release: add runc.keyring file and script

In order to allow any of the maintainers to cut releases for runc,
create a keyring file that distributions can use to verify that releases
are signed by one of the maintainers.

The format matches the gpg-offline format used by openSUSE packaging,
but it can be easily imported with "gpg --import" so any distribution
should be able to handle this keyring format wtihout issues.

Each key includes the GitHub handle of the associated user. There isn't
any way for this information to be automatically verified (outside of
using something like keybase.io) but since all changes of this file need
to be approved by maintainers this is okay for now.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

README.md

@@ -15,6 +15,8 @@
 
 You can find official releases of `runc` on the [release](https://github.com/opencontainers/runc/releases) page.
 
+All releases are signed by one of the keys listed in the [`runc.keyring` file in the root of this repository](runc.keyring).
+
 ## Security
 
 The reporting process and disclosure communications are outlined [here](https://github.com/opencontainers/org/blob/master/SECURITY.md).

script/keyring_addkey.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+# Copyright (C) 2023 SUSE LLC.
+# Copyright (C) 2023 Open Containers Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -Eeuxo pipefail
+
+root="$(readlink -f "$(dirname "${BASH_SOURCE[0]}")/..")"
+keyring_file="$root/runc.keyring"
+
+function bail() {
+	echo "$@" >&2
+	exit 1
+}
+
+[[ "$#" -eq 2 ]] || bail "usage: $0 <github-handle> <keyid>"
+
+github_handle="${1}"
+gpg_keyid="${2}"
+
+cat >>"$keyring_file" <<EOF
+$(gpg --list-keys "$gpg_keyid")
+
+$(gpg --armor --comment="github=$github_handle" --export --export-options=export-minimal,export-clean "$gpg_keyid")
+
+EOF