chore: import upstream snapshot with attribution

Sources/ContainerPlugin/ApplicationRoot.swift

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the container project 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
+//
+//   https://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.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import SystemPackage
+
+/// Provides the application data root path.
+public struct ApplicationRoot {
+    /// The environment variable that if set, determines the root directory for the application data store.
+    /// Otherwise, the system uses the default "~/Library/Application Support/com.apple.container".
+    public static let environmentName = "CONTAINER_APP_ROOT"
+
+    /// The default root directory used when ``environmentName`` is not set:
+    /// `~/Library/Application Support/com.apple.container`.
+    public static let defaultPath = FilePath(
+        FileManager.default.urls(
+            for: .applicationSupportDirectory,
+            in: .userDomainMask
+        ).first!.path(percentEncoded: false)
+    )
+    .appending(FilePath.Component("com.apple.container"))
+
+    /// The resolved root directory path, always lexically normalized.
+    ///
+    /// If the environment variable is set to an absolute path, that path is used directly.
+    /// If it is set to a relative path, the path is resolved against the working directory.
+    /// Otherwise, ``defaultPath`` is used.
+    public static let path = FilePath(FileManager.default.currentDirectoryPath).resolve(
+        ProcessInfo.processInfo.environment[environmentName],
+        defaultPath: defaultPath
+    )
+
+    /// The pathname to the root directory
+    public static let pathname = path.string
+}