chore: import upstream snapshot with attribution

Sources/ContainerPlugin/InstallRoot.swift

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+// 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 ContainerVersion
+import Foundation
+import SystemPackage
+
+/// Provides the application installation root path.
+public struct InstallRoot {
+    /// The environment variable that if set, determines the root directory for installed application.
+    /// Otherwise, the system computes the install path as the parent of the directory containing the
+    /// application binary (for example, "/usr/local/bin/container" -> "/usr/local").
+    public static let environmentName = "CONTAINER_INSTALL_ROOT"
+
+    /// The default root directory used when the environment variable is not set.
+    ///
+    /// Computed as the grandparent of ``CommandLine/executablePath``
+    /// (for example, `/usr/local/bin/container` → `/usr/local`).
+    /// Lexically normalized but not canonical, as symlinks in the executable path are not resolved.
+    public static let defaultPath = CommandLine.executablePath
+        .removingLastComponent()
+        .removingLastComponent()
+
+    /// 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
+}