Update build script in order to compile png images

I found the solution in the 2nd step of this PR rails/jsbundling-rails#58

Author: MartinSugasti

package.json

@@ -17,7 +17,7 @@
     "sass": "^1.65.1"
   },
   "scripts": {
-    "build": "esbuild app/javascript/*.* --bundle --sourcemap --outdir=app/assets/builds --public-path=/assets --loader:.js=jsx",
+    "build": "esbuild app/javascript/*.* --bundle --sourcemap --outdir=app/assets/builds --public-path=/assets --loader:.js=jsx --loader:.png=file",
     "build:css:compile": "sass ./app/assets/stylesheets/application.bootstrap.scss:./app/assets/builds/application.css --no-source-map --load-path=node_modules",
     "build:css:prefix": "postcss ./app/assets/builds/application.css --use=autoprefixer --output=./app/assets/builds/application.css",
     "build:css": "yarn build:css:compile && yarn build:css:prefix",