Report missing Docker clearly and preserve CI attempt setup

ci/Ci.java

@@ -25,7 +25,7 @@ public class Ci {
                     Path file = Path.of(Setup.required(env, "GITHUB_ENV"));
                     String id = DateTimeFormatter.ofPattern("yyyyMMddHHmmssSSSSSSSSS")
                             .withZone(ZoneOffset.UTC).format(Instant.now());
-                    Files.writeString(file, "CI_RUN_ATTEMPT=" + id + "\n", StandardOpenOption.APPEND);
+                    Files.writeString(file, "CI_RUN_ATTEMPT=" + id + "\n", StandardOpenOption.CREATE, StandardOpenOption.APPEND);
                 }
                 case "configure-gitea" -> Setup.configureGitea(env);
                 case "configure-docker" -> {

ci/CiTests.java

@@ -82,6 +82,14 @@ class CiTests {
         test("Older commit cannot move main", () -> {
             try (Fixture f = new Fixture()) { f.fake.head = "c".repeat(40); f.pipeline.publish(); equal(2L,f.calls("push")); }
         });
+        test("Unavailable Docker fails before build with a concrete cause", () -> {
+            try (Fixture f = new Fixture()) {
+                f.fake.dockerAvailable = false;
+                try { f.pipeline.verify(); throw new AssertionError("Expected failure"); }
+                catch (IllegalArgumentException error) { equal(true,error.getMessage().contains("Docker is unavailable")); }
+                equal(0L,f.fake.calls.stream().filter(a -> a.getFirst().equals("mvn")).count());
+            }
+        });
         test("Architecture mismatch blocks publication", () -> {
             try (Fixture f = new Fixture()) { f.pipeline.env.put("TARGET_PLATFORM","linux/arm64"); failure(f.pipeline::publish); equal(0L,f.calls("push")); }
         });
@@ -128,6 +136,7 @@ class CiTests {
         final Map<String,String> images = new HashMap<>(), tags = new HashMap<>();
         String head = SHA, pushFailure = "", pullFailure = "";
         boolean manifestError, mavenFailure;
+        boolean dockerAvailable = true;
         FakeCommands() { super(Duration.ofMinutes(1)); }
         @Override Result execute(List<String> args, Path cwd, Map<String,String> env, String input, boolean echo) {
             calls.add(List.copyOf(args));
@@ -135,7 +144,7 @@ class CiTests {
             if (args.getFirst().equals("mvn") && mavenFailure) return new Result(42,"deliberate failure");
             if (!args.getFirst().equals("docker")) return new Result(0,"");
             String command = args.get(1), ref = args.getLast();
-            if (command.equals("info")) return new Result(0,args.contains("{{.Architecture}}") ? "amd64" : "linux");
+            if (command.equals("info")) return new Result(dockerAvailable ? 0 : 1,args.contains("{{.Architecture}}") ? "amd64" : "linux");
             if (command.equals("context")) return new Result(0,"unix:///tmp/test-docker.sock");
             if (command.equals("manifest")) {
                 if (manifestError) return new Result(1,"unauthorized");

ci/Pipeline.java

@@ -40,7 +40,9 @@ class Pipeline {
     void run(String... args) throws Exception { result(true, args).checked(); }
     void requirePlatform() throws Exception {
         String target = Setup.required(env,"TARGET_PLATFORM");
-        String arch = capture("docker", "info", "--format", "{{.Architecture}}");
+        var docker = result(false,"docker", "info", "--format", "{{.Architecture}}");
+        check(docker.code() == 0, "Docker is unavailable; start Docker Desktop before running CI (details in verify.log)");
+        String arch = docker.output().strip();
         arch = switch (arch) { case "x86_64" -> "amd64"; case "aarch64" -> "arm64"; default -> arch; };
         String nativePlatform = capture("docker", "info", "--format", "{{.OSType}}") + "/" + arch;
         check(target.equals(nativePlatform), "Target " + target + " differs from Docker host " + nativePlatform);

docs/s4-validation.md

@@ -111,7 +111,7 @@ only then can real main publication/pullback/rerun acceptance be completed.
 
 CI orchestration, setup and behavior tests now run through `java ci/Ci.java`.
 The shell/Python helpers have been removed. The source launcher compiles helper
-classes in memory without external dependencies or preview flags. Fourteen Java
+classes in memory without external dependencies or preview flags. Fifteen Java
 behavior tests pass, including JSON/configuration preservation, publication gates,
 partial recovery and process deadlines. A real feature run validates this revision
 before updating the existing open PR; no merge to main is authorized.