backport: surface docker errors and add --debug flag for worker logs

Cherry-pick of KeygraphHQ/shannon#299 (ccb5303).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/cli/src/commands/start.ts

@@ -22,6 +22,7 @@ export interface StartArgs {
   workspace?: string;
   output?: string;
   pipelineTesting: boolean;
+  debug: boolean;
   version: string;
 }
 
@@ -110,14 +111,22 @@ export async function start(args: StartArgs): Promise<void> {
     ...(outputDir && { outputDir }),
     workspace,
     ...(args.pipelineTesting && { pipelineTesting: true }),
+    ...(args.debug && { debug: true }),
   });
 
-  // 14. Wait for workflow to register, then display info
-  proc.on('error', (err) => {
-    console.error(`Failed to start worker: ${err.message}`);
-    process.exit(1);
+  // 14. Bail if `docker run -d` itself fails (mount error, image missing, etc.)
+  const dockerExitCode = await new Promise<number>((resolve) => {
+    proc.once('exit', (code) => resolve(code ?? 1));
+    proc.once('error', (err) => {
+      console.error(`Failed to start worker: ${err.message}`);
+      resolve(1);
+    });
   });
 
+  if (dockerExitCode !== 0) {
+    process.exit(1);
+  }
+
   // Detect whether this is a fresh workspace or a resume by checking session.json existence
   const sessionJson = path.join(workspacesDir, workspace, 'session.json');
   const isResume = fs.existsSync(sessionJson);
@@ -182,6 +191,9 @@ export async function start(args: StartArgs): Promise<void> {
     } catch {
       // Container may have already exited
     }
+    if (args.debug) {
+      printDebugHint(containerName);
+    }
   };
 
   process.on('SIGINT', () => {
@@ -195,6 +207,14 @@ export async function start(args: StartArgs): Promise<void> {
   process.on('exit', cleanup);
 }
 
+function printDebugHint(containerName: string): void {
+  console.log('');
+  console.log(`  Worker container preserved: ${containerName}`);
+  console.log(`    Inspect logs: docker logs ${containerName}`);
+  console.log(`    Remove:       docker rm ${containerName}`);
+  console.log('');
+}
+
 function printInfo(
   args: StartArgs,
   workspace: string,

apps/cli/src/docker.ts

@@ -159,13 +159,19 @@ export interface WorkerOptions {
   outputDir?: string;
   workspace: string;
   pipelineTesting?: boolean;
+  debug?: boolean;
 }
 
 /**
  * Spawn the worker container in detached mode and return the process.
+ * When `opts.debug` is true, omits `--rm` so the container persists for log inspection.
  */
 export function spawnWorker(opts: WorkerOptions): ChildProcess {
-  const args = ['run', '-d', '--rm', '--name', opts.containerName, '--network', 'shannon-net'];
+  const args = ['run', '-d'];
+  if (!opts.debug) {
+    args.push('--rm');
+  }
+  args.push('--name', opts.containerName, '--network', 'shannon-net');
 
   // Add host flag for Linux
   args.push(...addHostFlag());
@@ -227,9 +233,11 @@ export function spawnWorker(opts: WorkerOptions): ChildProcess {
     args.push('--pipeline-testing');
   }
 
-  // Prevent MSYS/Git Bash from converting Unix paths (e.g. /repos/my-repo) to Windows paths
+  // Inherit stderr so `docker run` daemon errors surface to the user;
+  // ignore stdin/stdout (the container ID is noise).
   return spawn('docker', args, {
-    stdio: 'pipe',
+    stdio: ['ignore', 'ignore', 'inherit'],
+    // Prevent MSYS/Git Bash from converting Unix paths on Windows
     ...(os.platform() === 'win32' && { env: { ...process.env, MSYS_NO_PATHCONV: '1' } }),
   });
 }

apps/cli/src/index.ts

@@ -70,6 +70,7 @@ Options for 'start':
   -o, --output <path>       Copy deliverables to this directory after run
   -w, --workspace <name>    Named workspace (auto-resumes if exists)
       --pipeline-testing    Use minimal prompts for fast testing
+      --debug               Preserve worker container after exit for log inspection
 
 Examples:
   ${prefix} start -u https://example.com -r ${mode === 'local' ? 'my-repo' : './my-repo'}
@@ -94,6 +95,7 @@ interface ParsedStartArgs {
   workspace?: string;
   output?: string;
   pipelineTesting: boolean;
+  debug: boolean;
 }
 
 function parseStartArgs(argv: string[]): ParsedStartArgs {
@@ -103,6 +105,7 @@ function parseStartArgs(argv: string[]): ParsedStartArgs {
   let workspace: string | undefined;
   let output: string | undefined;
   let pipelineTesting = false;
+  let debug = false;
 
   for (let i = 0; i < argv.length; i++) {
     const arg = argv[i];
@@ -147,6 +150,9 @@ function parseStartArgs(argv: string[]): ParsedStartArgs {
       case '--pipeline-testing':
         pipelineTesting = true;
         break;
+      case '--debug':
+        debug = true;
+        break;
       default:
         console.error(`Unknown option: ${arg}`);
         console.error(`Run "${getMode() === 'local' ? './shannon' : 'npx @keygraph/shannon'} help" for usage`);
@@ -164,6 +170,7 @@ function parseStartArgs(argv: string[]): ParsedStartArgs {
     url,
     repo,
     pipelineTesting,
+    debug,
     ...(config && { config }),
     ...(workspace && { workspace }),
     ...(output && { output }),