Tweaks to the frontend build in the Dockerfile

A couple of related improvements to how we build the Dockerfile:

1. Don't invalidate the Python dependencies layer just because
   `package.json`, `gulpfile.js` or `scripts/` changed.
2. Use a prebuilt node-sass binary. The prebuilt binaries supplied by
   the node-sass project aren't compatible with Alpine Linux (which uses
   musl-libc rather than glibc).
3. Make some small tweaks to the gulpfile so we can build just the
   assets needed for the web application (i.e. skipping the extension).

gulpfile.js

@@ -12,7 +12,6 @@ var gulpIf = require('gulp-if');
 var gulpUtil = require('gulp-util');
 var sass = require('gulp-sass');
 var postcss = require('gulp-postcss');
-var runSequence = require('run-sequence');
 var sourcemaps = require('gulp-sourcemaps');
 
 var manifest = require('./scripts/gulp/manifest');
@@ -62,10 +61,6 @@ var appBundles = [{
   name: 'app',
   transforms: ['coffee'],
   entry: './h/static/scripts/app.coffee',
-},{
-  // Browser extension background script
-  name: 'extension',
-  entry: './h/browser/chrome/lib/extension',
 },{
   // The Annotator library which provides annotation controls on
   // the page and sets up the sidebar
@@ -102,6 +97,23 @@ gulp.task('watch-app-js', ['build-vendor-js'], function () {
   });
 });
 
+var extensionBundleConfig = {
+  name: 'extension',
+  entry: './h/browser/chrome/lib/extension',
+  path: SCRIPT_DIR,
+  external: vendorModules,
+  minify: IS_PRODUCTION_BUILD,
+  noParse: vendorBundles.noParseModules,
+};
+
+gulp.task('build-extension-js', ['build-vendor-js'], function () {
+  return createBundle(extensionBundleConfig);
+});
+
+gulp.task('watch-extension-js', ['build-vendor-js'], function () {
+  return createBundle(extensionBundleConfig, {watch: true});
+});
+
 var styleFiles = [
   // H
   './h/static/styles/admin.scss',
@@ -174,8 +186,6 @@ function generateManifest() {
   });
 }
 
-gulp.task('generate-manifest', generateManifest);
-
 gulp.task('watch-manifest', function () {
   gulp.watch(MANIFEST_SOURCE_FILES, batch(function (events, done) {
     endOfStream(generateManifest(), function () {
@@ -184,13 +194,25 @@ gulp.task('watch-manifest', function () {
   }));
 });
 
-gulp.task('build', function (callback) {
-  runSequence(['build-app-js', 'build-css',
-               'build-fonts', 'build-images'],
-              'generate-manifest',
-              callback);
-});
-
-gulp.task('watch', ['watch-app-js', 'watch-css',
-                    'watch-fonts', 'watch-images',
+gulp.task('build-app',
+          ['build-app-js',
+           'build-css',
+           'build-fonts',
+           'build-images'],
+          generateManifest);
+
+gulp.task('build',
+          ['build-app-js',
+           'build-extension-js',
+           'build-css',
+           'build-fonts',
+           'build-images'],
+          generateManifest);
+
+gulp.task('watch',
+          ['watch-app-js',
+           'watch-extension-js',
+           'watch-css',
+           'watch-fonts',
+           'watch-images',
            'watch-manifest']);

package.json

@@ -55,7 +55,6 @@
     "raf": "^3.1.0",
     "raven-js": "^2.0.2",
     "retry": "^0.8.0",
-    "run-sequence": "^1.1.5",
     "scroll-into-view": "^1.3.1",
     "showdown": "^1.2.1",
     "through2": "^2.0.1",
@@ -88,9 +87,6 @@
   "engines": {
     "node": "0.10.x"
   },
-  "scripts": {
-    "build-assets": "gulp build"
-  },
   "repository": {
     "type": "git",
     "url": "https://github.com/hypothesis/h.git"