add gd extension guide

docs/.vuepress/config.js

@@ -74,6 +74,7 @@ module.exports = {
           '/guides/object-cache',
           '/guides/cloudflare',
           '/guides/new-relic',
+          '/guides/gd-extension',
         ]
       },
       {

docs/guides/gd-extension.md

@@ -0,0 +1,45 @@
+# How to install the GD PHP extension
+
+[[toc]]
+
+## Why use the GD PHP extension?
+
+By default, the Ymir runtime comes with the [Imagick PHP extension][1] for image processing. In contrast to the [GD extension][2], the Imagick extension has more features and capabilities. This is why Ymir uses it as its image processing extension.
+
+However, there are some plugins that only support the GD extension. For that reason, you'll need to install the GD extension in your project.
+
+## Requirements
+
+To install the GD extension in your Ymir project, you'll need to:
+
+ * Use [container image deployment][3]
+ * Use `x86_64` as your [`architecture`][4]
+
+## Configuring your Ymir project to install the GD extension
+
+Once your project satisfies the above requirements, you'll need to replace your project's `Dockerfile` with the one below. Please note that `<PHP_VERSION>` is a placeholder. You should replace it with your desired PHP version. (e.g. `81` for PHP 8.1)
+
+```docker
+FROM --platform=linux/x86_64 ymirapp/php-runtime:php-<PHP_VERSION>
+
+ENTRYPOINT []
+
+CMD ["/bin/sh", "-c", "/opt/bootstrap"]
+
+RUN LD_LIBRARY_PATH= yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
+RUN LD_LIBRARY_PATH= yum install -y https://rpms.remirepo.net/enterprise/remi-release-7.rpm
+RUN LD_LIBRARY_PATH= yum install -y yum-utils
+RUN LD_LIBRARY_PATH= yum-config-manager --enable remi-php<PHP_VERSION>
+RUN LD_LIBRARY_PATH= yum install -y php<PHP_VERSION>-php-gd
+
+RUN echo "extension=/opt/remi/php<PHP_VERSION>/root/usr/lib64/php/modules/gd.so" >> /opt/ymir/etc/php/conf.d/php.ini
+
+COPY runtime/src /opt/src
+
+COPY . /var/task
+```
+
+[1]: https://www.php.net/manual/en/book.imagick.php
+[2]: https://www.php.net/manual/en/book.image.php
+[3]: ./container-image-deployment.md
+[4]: ../reference/configuration.md#architecture