generator: Add an environment variable to choose the container runtime

Author: euanh

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator+Copy.swift

@@ -18,7 +18,7 @@ extension SwiftSDKGenerator {
     baseDockerImage: String,
     sdkDirPath: FilePath
   ) async throws {
-    logGenerationStep("Launching a Docker container to copy Swift SDK for the target triple from it...")
+    logGenerationStep("Launching a container to extract the Swift SDK for the target triple...")
     try await withDockerContainer(fromImage: baseDockerImage) { containerID in
       try await inTemporaryDirectory { generator, _ in
         let sdkUsrPath = sdkDirPath.appending("usr")

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator.swift

@@ -60,7 +60,7 @@ public actor SwiftSDKGenerator {
   }
 
   private let fileManager = FileManager.default
-  private static let dockerCommand = "docker"
+  private static let dockerCommand = ProcessInfo.processInfo.environment["SWIFT_SDK_GENERATOR_CONTAINER_RUNTIME"] ?? "docker"
 
   public static func getCurrentTriple(isVerbose: Bool) throws -> Triple {
     let current = UnixName.current!