feat: Add remote image caching behaviour docs

src/content/docs/en/guides/images.mdx

@@ -28,7 +28,7 @@ Store your images in the `public/` folder if you want to avoid any processing or
 You can also choose to store your images remotely, in a [content management system (CMS)](/en/guides/cms/) or [digital asset management (DAM)](/en/guides/media/) platform.
 Astro can fetch your data remotely using APIs or display images from their full URL path.
 
- For extra protection when dealing with external sources, Astro's image components and helper function will only process (e.g. optimize, transform) images from [authorized image sources specified in your configuration](#authorizing-remote-images). Remote images from other sources will be displayed with no processing.
+For extra protection when dealing with external sources, Astro's image components and helper function will only process (e.g. optimize, transform) images from [authorized image sources specified in your configuration](#authorizing-remote-images). Remote images from other sources will be displayed with no processing.
 
 ## Images in `.astro` files
 
@@ -475,6 +475,25 @@ export default defineConfig({
 });
 ```
 
+## Asset Caching
+
+Astro stores processed image assets in a cache directory during site builds for both local and [remote images from authorized sources](/en/guides/images/#authorizing-remote-images). By preserving the cache directory between builds, processed assets are reused, improving build time and bandwidth usage.
+
+The default cache directory is `./node_modules/.astro`, however this can be changed using the [`cacheDir`](/en/reference/configuration-reference/#cachedir) configuration setting.
+
+### Remote Images
+
+Remote images in the asset cache are managed based on [HTTP Caching](https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching), and respect the [Cache-Control header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control) returned by the remote server.
+Images are cached if the Cache-Control header allows, and will be used until they are no longer [fresh](https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#fresh_and_stale_based_on_age).
+
+#### Revalidation
+
+<p><Since v="5.1.0" /></p>
+
+[Revalidation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#validation) reduces bandwidth usage and build time by checking with the remote server whether an expired cached image is still up-to-date. If the server indicates that the image is still fresh, the cached version is reused, otherwise the image is redownloaded.
+
+Revalidation requires that the remote server send [Last-Modified](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified) and/or [Etag (entity tag)](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) headers with its responses. This feature is available for remote servers that support the [If-Modified-Since](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since) and [If-None-Match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match) headers.
+
 ## Community Integrations
 
 There are several third-party [community image integrations](https://astro.build/integrations?search=images) for optimizing and working with images in your Astro project.