Build wardrobe-2020 JS during dev container setup

Oh yeah right, let's `yarn install` and build as part of the `post-create.sh` script!

I didn't have a command to do a one-time dev build of the assets yet (just one-time prod, or dev with reloading), so I added `yarn build:dev`!

.devcontainer/post-create.sh

@@ -11,3 +11,9 @@ bundle install
 
 # Set up the databases: create the schema, and load in some default data.
 bin/rails db:schema:load db:seed
+
+# Install the app's JS dependencies.
+yarn install
+
+# Run a first-time build of the app's JS, in development mode.
+yarn build:dev

package.json

@@ -31,6 +31,7 @@
   },
   "scripts": {
     "build": "esbuild app/javascript/*.* --bundle --sourcemap --outdir=app/assets/builds --public-path=/assets --asset-names=[name]-[hash].digested --loader:.js=jsx --loader:.png=file --loader:.svg=file --loader:.min.js=text",
-    "dev": "yarn build --watch --public-path=/dev-assets"
+    "build:dev": "yarn build --public-path=/dev-assets",
+    "dev": "yarn build:dev --watch"
   }
 }