cli: add skeleton tarball for docker build caching

packages/backend/Dockerfile

@@ -2,10 +2,15 @@ FROM node:12
 
 WORKDIR /usr/src/app
 
+# Copy repo skeleton first, to avoid unnecessary docker cache invalidation.
+# The skeleton contains the package.json of each package in the monorepo,
+# and along with yarn.lock and the root package.json, that's enough to run yarn install.
+ADD yarn.lock package.json skeleton.tar ./
+
+RUN yarn install --frozen-lockfile --production
+
 # This will copy the contents of the dist-workspace when running the build-image command.
 # Do not use this Dockerfile outside of that command, as it will copy in the source code instead.
 COPY . .
 
-RUN yarn install --frozen-lockfile --production
-
 CMD ["node", "packages/backend"]

packages/cli/src/commands/backend/buildImage.ts

@@ -41,6 +41,7 @@ export default async (cmd: Command) => {
       ...appConfigs,
       { src: paths.resolveTarget('Dockerfile'), dest: 'Dockerfile' },
     ],
+    skeleton: 'skeleton.tar',
   });
   console.log(`Dist workspace ready at ${tempDistWorkspace}`);
 

packages/cli/src/lib/packager/index.ts

@@ -15,10 +15,14 @@
  */
 
 import fs from 'fs-extra';
-import { resolve as resolvePath, relative as relativePath } from 'path';
+import {
+  join as joinPath,
+  resolve as resolvePath,
+  relative as relativePath,
+} from 'path';
 import { paths } from '../paths';
 import { run } from '../run';
-import tar from 'tar';
+import tar, { CreateOptions } from 'tar';
 import { tmpdir } from 'os';
 
 type LernaPackage = {
@@ -53,6 +57,12 @@ type Options = {
    * If set to true, the target packages are built before they are packaged into the workspace.
    */
   buildDependencies?: boolean;
+
+  /**
+   * If set, creates a skeleton tarball that contains all package.json files
+   * with the same structure as the workspace dir.
+   */
+  skeleton?: 'skeleton.tar';
 };
 
 /**
@@ -89,6 +99,24 @@ export async function createDistWorkspace(
     const dest = typeof file === 'string' ? file : file.dest;
     await fs.copy(paths.resolveTargetRoot(src), resolvePath(targetDir, dest));
   }
+
+  if (options.skeleton) {
+    const skeletonFiles = targets.map(target => {
+      const dir = relativePath(paths.targetRoot, target.location);
+      return joinPath(dir, 'package.json');
+    });
+
+    await tar.create(
+      {
+        file: resolvePath(targetDir, options.skeleton),
+        cwd: targetDir,
+        portable: true,
+        noMtime: true,
+      } as CreateOptions & { noMtime: boolean },
+      skeletonFiles,
+    );
+  }
+
   return targetDir;
 }