Use v5 language mode for containertool (#6)

* containertool: Switch to Swift 5 language mode

The nightly mainline compiler used by the pull request checks objects
to the way containertool uses stderr.   Switch to language mode 5 until
this can be rewritten.

* ci: Enable API breakage checks

* docs: Generate documentation for containertool

Author: euanh

.github/workflows/pull_request.yml

@@ -10,7 +10,6 @@ jobs:
         uses: apple/swift-nio/.github/workflows/soundness.yml@main
         with:
             api_breakage_check_container_image: "swift:6.0-noble"
-            api_breakage_check_enabled: false
             docs_check_container_image: "swift:6.0-noble"
             license_header_check_project_name: "SwiftContainerPlugin"
             shell_check_container_image: "swift:6.0-noble"

.spi.yml

@@ -2,5 +2,5 @@ version: 1
 builder:
   configs:
   - documentation_targets:
-    - ContainerRegistry
+    - containertool
     - ContainerImageBuilderPlugin

Package.swift

@@ -43,7 +43,8 @@ let package = Package(
                 .product(name: "ArgumentParser", package: "swift-argument-parser"),
                 .product(name: "Crypto", package: "swift-crypto", condition: .when(platforms: [.linux])),
                 .target(name: "ContainerRegistry"), .target(name: "VendorCNIOExtrasZlib"),
-            ]
+            ],
+            swiftSettings: [.swiftLanguageMode(.v5)]
         ),
         .target(
             // Vendored from https://github.com/apple/swift-nio-extras