🎉 Use build bind mounts to avoid extra Docker layers

lib/dockerfile.js

@@ -49,22 +49,24 @@ export function generateDockerfile (options) {
     lines.push('WORKDIR /var/task')
   }
 
-  // Copy stripped package files
-  // These files have the version stripped, so that docker can cache better.
-  // Stripping is not needed on the yarn lockfile, as that doesn't include pacakge version.
-  lines.push(`COPY ${directoryPrefix}scandium-clean-package.json package.json`)
-  lines.push(`COPY ${directoryPrefix}${yarn ? 'yarn.lock yarn.lock' : 'scandium-clean-package-lock.json package-lock.json'}`)
-
   // Add build argument for SSH key, and prime known_hosts with GitHub public key
   if (sshKey) lines.push('ARG SSH_PRIVATE_KEY', 'ARG SSH_PUBLIC_KEY', `RUN mkdir -p $HOME/.ssh && echo "${githubPublicKey}" >> $HOME/.ssh/known_hosts`)
 
+  // Mount stripped package files
+  // These files have the version stripped, so that docker can cache better.
+  // Stripping is not needed on the yarn lockfile, as that doesn't include pacakge version.
+  const packageFileMounts = [
+    `--mount=type=bind,source=${directoryPrefix}scandium-clean-package.json,target=package.json`,
+    `--mount=type=bind,source=${directoryPrefix}${yarn ? 'yarn.lock' : 'scandium-clean-package-lock.json'},target=${yarn ? 'yarn.lock' : 'package-lock.json'}`
+  ].join(' ')
+
   // Add production dependencies
   // This step is run before adding the code, to increase docker cache use.
-  if (hasProductionDependencies && !skipNodeModules) lines.push(`RUN ${sshKey ? sshPrefix : ''}${yarn ? 'yarn install --production --frozen-lockfile' : 'npm ci --production'}${sshKey ? sshPostfix : ''}`)
+  if (hasProductionDependencies && !skipNodeModules) lines.push(`RUN ${packageFileMounts} ${sshKey ? sshPrefix : ''}${yarn ? 'yarn install --production --frozen-lockfile' : 'npm ci --production'}${sshKey ? sshPostfix : ''}`)
   if (hasProductionDependencies && !skipNodeModules) lines.push('RUN zip -9qyr /output.zip node_modules')
 
   // Install dev-dependencies, if there is a `prepare` or `build` script present
-  if (hasPrepareScript || hasBuildScript) lines.push(`RUN ${sshKey ? sshPrefix : ''}${yarn ? 'yarn install --frozen-lockfile' : 'npm ci'}${sshKey ? sshPostfix : ''}`)
+  if (hasPrepareScript || hasBuildScript) lines.push(`RUN ${packageFileMounts} ${sshKey ? sshPrefix : ''}${yarn ? 'yarn install --frozen-lockfile' : 'npm ci'}${sshKey ? sshPostfix : ''}`)
 
   // Add the app files, and remove our special files
   if (options.directory) {