config to start docker and runner at startup

ci/runner/WaitForDocker.java

@@ -0,0 +1,35 @@
+import java.time.Duration;
+import java.util.concurrent.TimeUnit;
+
+/** Source-launcher startup probe; uses the service's configured DOCKER_HOST. */
+class WaitForDocker {
+    public static void main(String[] args) throws Exception {
+        long seconds = args.length == 0 ? 120 : Long.parseLong(args[0]);
+        if (seconds < 1 || seconds > 300) throw new IllegalArgumentException("Timeout must be 1–300 seconds");
+        long deadline = System.nanoTime() + Duration.ofSeconds(seconds).toNanos();
+        System.out.println("Waiting for the configured Docker engine...");
+        while (System.nanoTime() < deadline) {
+            Process process = new ProcessBuilder("docker", "info", "--format", "{{.ServerVersion}}")
+                    .redirectOutput(ProcessBuilder.Redirect.DISCARD)
+                    .redirectError(ProcessBuilder.Redirect.DISCARD)
+                    .start();
+            try {
+                long remaining = Math.max(1, deadline - System.nanoTime());
+                if (process.waitFor(Math.min(remaining, TimeUnit.SECONDS.toNanos(5)), TimeUnit.NANOSECONDS)
+                        && process.exitValue() == 0) {
+                    System.out.println("Docker is available; starting the Gitea runner.");
+                    return;
+                }
+            } finally {
+                if (process.isAlive()) {
+                    process.destroyForcibly();
+                    process.waitFor();
+                }
+            }
+            long remaining = deadline - System.nanoTime();
+            if (remaining > 0) TimeUnit.NANOSECONDS.sleep(Math.min(remaining, TimeUnit.SECONDS.toNanos(2)));
+        }
+        System.err.println("Docker did not become available within " + seconds + " seconds; runner not started.");
+        System.exit(1);
+    }
+}