Introduce Apptainer's image cache explanation

Added section on Apptainer's image cache and its benefits.

Author: Amanda-dong

docs/hpc/15_tutorial_apptainer/01_intro_apptainer.mdx

@@ -5,6 +5,7 @@ Researchers often rely on complex software stacks that include programming langu
 
 Containers provide a way to package software together with the environment it needs to run. Instead of manually installing every dependency on a system, users can run software inside a container that already includes the required libraries and tools.
 
+
 ## What is Apptainer?
 Apptainer is a container platform that allows users to run software inside isolated environments without requiring administrator privileges on the host system.
 
@@ -23,3 +24,9 @@ Torch provides container images with both `.sif` and `.sqf` extensions.
 Some Torch-provided application images use `.sqf` files and are typically accessed through wrapper scripts such as `run-anaconda3-2024.10-1.bash`. These scripts handle the details of launching the application environment and are often the recommended interface for users.
 
 When available, use the wrapper script documented for that application. For general Apptainer examples in this tutorial, we use `.sif` images because they can be launched directly with standard Apptainer commands.
+
+## Apptainer’s image cache
+
+While Apptainer doesn’t have a local image repository in the same way as Docker, it does cache downloaded image files. As we saw in the previous episode, images are simply `.sif` files stored on your local disk.
+
+If you delete a local `.sif` image that you have pulled from a remote image repository and then pull it again, if the image is unchanged from the version you previously pulled, you will be given a copy of the image file from your local cache rather than the image being downloaded again from the remote source. This removes unnecessary network transfers and is particularly useful for large images which may take some time to transfer over the network.