Add simple instructions to providing an NPM Token to backend:build-image

packages/cli/src/commands/index.ts

@@ -55,7 +55,9 @@ export function registerCommands(program: CommanderStatic) {
     .helpOption(', --backstage-cli-help') // Let docker handle --help
     .option('--build', 'Build packages before packing them into the image')
     .description(
-      'Bundles the package into a docker image. All extra args are forwarded to docker image build',
+      'Bundles the package into a docker image. All extra args are forwarded to ' +
+        '`docker image build`. For example, if a $NPM_TOKEN needs to be exposed, run ' +
+        '`backend:build-image --secret id=NPM_TOKEN,src=/NPM_TOKEN.txt`',
     )
     .action(lazy(() => import('./backend/buildImage').then(m => m.default)));