fix: use correct platform when creating remote buildx builder

The remote builder was hardcoded to use --platform linux/amd64
regardless of user input or runner architecture. This caused
performance issues on ARM runners and cache inefficiencies.

Now properly uses the platforms input or detects host architecture
to avoid unnecessary QEMU emulation and improve build performance.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -40,7 +40,7 @@
   },
   "devDependencies": {
     "@babel/core": "^7.26.10",
-    "@babel/preset-env": "^7.26.9",
+    "@babel/preset-env": "^7.27.2",
     "@babel/preset-typescript": "^7.27.0",
     "@types/jest": "^29.5.14",
     "@types/node": "^20.12.12",

src/__mocks__/@buf/blacksmith_vm-agent.bufbuild_es/stickydisk/v1/stickydisk_pb.ts

@@ -9,4 +9,4 @@ export class Metric {
   type: Metric_MetricType = Metric_MetricType.UNSPECIFIED;
   value: number = 0;
   labels: Record<string, string> = {};
-} 
+}

src/__mocks__/@buf/blacksmith_vm-agent.connectrpc_es/stickydisk/v1/stickydisk_connect.ts

@@ -1,23 +1,23 @@
 export class StickyDiskService {
   constructor() {}
-  
+
   async commitStickyDisk() {
     return {};
   }
-  
+
   async getStickyDisk() {
     return {};
   }
-  
+
   async queueDockerJob() {
     return {};
   }
-  
+
   async reportMetric() {
     return {};
   }
-  
+
   async up() {
     return {};
   }
-} 
+}

src/__mocks__/execa.ts

@@ -4,4 +4,4 @@ export const execa = jest.fn().mockImplementation(() => {
     stderr: '',
     exitCode: 0
   };
-}); 
+});

src/__tests__/remote-builder-args.test.ts

@@ -0,0 +1,41 @@
+import * as os from 'os';
+import {getRemoteBuilderArgs, resolveRemoteBuilderPlatforms} from '../context';
+
+jest.mock('@actions/core', () => ({
+  info: jest.fn(),
+  debug: jest.fn(),
+  warning: jest.fn(),
+  error: jest.fn()
+}));
+
+describe('Remote builder platform argument resolution', () => {
+  const builderName = 'test-builder';
+  const builderUrl = 'tcp://127.0.0.1:1234';
+
+  afterEach(() => {
+    jest.restoreAllMocks();
+  });
+
+  test('returns comma-separated list when platforms are supplied', async () => {
+    const platforms = ['linux/arm64', 'linux/amd64'];
+    const platformStr = resolveRemoteBuilderPlatforms(platforms);
+    expect(platformStr).toBe('linux/arm64,linux/amd64');
+
+    const args = await getRemoteBuilderArgs(builderName, builderUrl, platforms);
+    const idx = args.indexOf('--platform');
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe('linux/arm64,linux/amd64');
+  });
+
+  test('falls back to host architecture when no platforms supplied', async () => {
+    jest.spyOn(os, 'arch').mockReturnValue('arm64' as any);
+
+    const platformStr = resolveRemoteBuilderPlatforms([]);
+    expect(platformStr).toBe('linux/arm64');
+
+    const args = await getRemoteBuilderArgs(builderName, builderUrl, []);
+    const idx = args.indexOf('--platform');
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe('linux/arm64');
+  });
+});

src/context.ts

@@ -1,6 +1,6 @@
 import * as core from '@actions/core';
 import * as handlebars from 'handlebars';
-import * as fs from 'fs';
+import * as os from 'os';
 
 import {Build} from '@docker/actions-toolkit/lib/buildx/build';
 import {Context} from '@docker/actions-toolkit/lib/context';
@@ -336,12 +336,39 @@ export const tlsClientKeyPath = '/tmp/blacksmith_client_key.pem';
 export const tlsClientCaCertificatePath = '/tmp/blacksmith_client_ca_certificate.pem';
 export const tlsRootCaCertificatePath = '/tmp/blacksmith_root_ca_certificate.pem';
 
-export async function getRemoteBuilderArgs(name: string, builderUrl: string): Promise<Array<string>> {
+/**
+ * Resolve the platform list that should be passed to `docker buildx create`.
+ *
+ * Priority:
+ *   1. Use the user-supplied platforms list (comma-joined) if provided.
+ *   2. Fallback to the architecture of the host runner.
+ *
+ * The function is exported to allow isolated unit testing.
+ */
+export function resolveRemoteBuilderPlatforms(platforms?: string[]): string {
+  // If user explicitly provided platforms, honour them verbatim.
+  if (platforms && platforms.length > 0) {
+    return platforms.join(',');
+  }
+
+  // Otherwise derive from host architecture.
+  const nodeArch = os.arch(); // e.g. 'x64', 'arm64', 'arm'
+  const archMap: {[key: string]: string} = {
+    x64: 'amd64',
+    arm64: 'arm64',
+    arm: 'arm'
+  };
+  const mappedArch = archMap[nodeArch] || nodeArch;
+  return `linux/${mappedArch}`;
+}
+
+export async function getRemoteBuilderArgs(name: string, builderUrl: string, platforms?: string[]): Promise<Array<string>> {
   const args: Array<string> = ['create', '--name', name, '--driver', 'remote'];
 
-  // TODO(aayush): Instead of hardcoding the platform, we should fail the build if the platform is
-  // unsupported.
-  args.push('--platform', 'linux/amd64');
+  const platformFlag = resolveRemoteBuilderPlatforms(platforms);
+  core.info(`Determined remote builder platform(s): ${platformFlag}`);
+  args.push('--platform', platformFlag);
+
   // Always use the remote builder, overriding whatever has been configured so far.
   args.push('--use');
   // Use the provided builder URL

src/main.ts

@@ -202,7 +202,7 @@ actionsToolkit.run(
       if (builderInfo.addr) {
         await core.group(`Creating a builder instance`, async () => {
           const name = `blacksmith-${Date.now().toString(36)}`;
-          const createCmd = await toolkit.buildx.getCommand(await context.getRemoteBuilderArgs(name, builderInfo.addr!));
+          const createCmd = await toolkit.buildx.getCommand(await context.getRemoteBuilderArgs(name, builderInfo.addr!, inputs.platforms));
           core.info(`Creating builder with command: ${createCmd.command}`);
           await Exec.getExecOutput(createCmd.command, createCmd.args, {
             ignoreReturnCode: true