chore: import upstream snapshot with attribution

Sources/ContainerBuild/BuildFile.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+// 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 Logging
+
+public struct BuildFile {
+    /// Tries to resolve either a Dockerfile or Containerfile relative to contextDir.
+    /// Checks for Dockerfile, then falls back to Containerfile.
+    public static func resolvePath(contextDir: String, log: Logger? = nil) throws -> String? {
+        // Check for Dockerfile then Containerfile in context directory
+        let dockerfilePath = URL(filePath: contextDir).appendingPathComponent("Dockerfile").path
+        let containerfilePath = URL(filePath: contextDir).appendingPathComponent("Containerfile").path
+
+        let dockerfileExists = FileManager.default.fileExists(atPath: dockerfilePath)
+        let containerfileExists = FileManager.default.fileExists(atPath: containerfilePath)
+
+        if dockerfileExists && containerfileExists {
+            log?.info("Detected both Dockerfile and Containerfile, choosing Dockerfile")
+            return dockerfilePath
+        }
+
+        if dockerfileExists {
+            return dockerfilePath
+        }
+
+        if containerfileExists {
+            return containerfilePath
+        }
+
+        return nil
+    }
+}