perf: update dockerfile to better version

Update Dockerfile to a better version that is slimmer and has no vulnerabilities detected by IBM
Cloud

Author: emonddr

packages/cli/generators/app/templates/Dockerfile

@@ -1,4 +1,5 @@
-FROM node:10
+# Check out https://hub.docker.com/_/node to select a new base image
+FROM node:10-slim
 
 # Set to a non-root built-in user `node`
 USER node

packages/cli/test/integration/generators/app.integration.js

@@ -105,7 +105,7 @@ describe('app-generator specific files', () => {
   });
 
   it('generates docker files', () => {
-    assert.fileContent('Dockerfile', /FROM node:10/);
+    assert.fileContent('Dockerfile', /FROM node:10-slim/);
     assert.fileContent('.dockerignore', /node_modules/);
 
     assert.fileContent('package.json', /"docker:build": "docker build/);