title: How store multiple sizes of same image with ActiveStorage?
body: <p>I have image uploads working like so</p>
  <pre class="lang-rb prettyprint-override"><code>#user.rb
  has_many_attached :images
  </code></pre>
  <pre class="lang-rb prettyprint-override"><code>&lt;%= f.label :user_images %&gt;
  &lt;%= f.file_field :user_images, multiple: true %&gt;
  </code></pre>
  <p>I know how to resize images <em>in the view</em> e.g. <code>image_tag(user.images.first, {size: &quot;300x300&quot;})</code></p>
  <p>The problem with this approach is that it sends the full-size image to the view, then resizes it using CSS, so it can cause a lot of unnecessary use of the network. So I prefer to store multiple sizes when the user uploads the image, then serve the appropriate sized image as required.</p>
  <p>How do I do this?</p>