Add Apptainer tutorial for Torch HPC cluster

This tutorial provides step-by-step instructions for using Apptainer on the Torch HPC cluster, including pulling a container image, running it, executing commands, and exploring the container environment.

Author: Amanda-dong

docs/hpc/07_containers/02_apptainer_tutorial

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+# Getting Started with Apptainer on Torch
+
+This tutorial walks you through running your first container on the Torch HPC cluster using Apptainer. By the end of this guide, you will be able to pull a container image, run it, and execute commands inside a containerized environment.
+
+## Step 1: Pull a Container Image
+
+Make sure you are on a compute node (via `srun --pty bash`) before proceeding, as running containers on login nodes is not recommended.
+
+Pull a container image from Docker Hub:
+
+```sh
+apptainer pull docker://python:3.10
+```
+
+This command downloads a Docker image and converts it into an Apptainer image (`.sif`) that can be run on the cluster.
+
+## Step 2: Run the Container
+
+You can run the container using:
+
+```sh
+apptainer run python_3.10.sif
+```
+
+This executes the default command defined by the container image.
+
+## Step 3: Execute Commands Inside the Container
+
+To run specific commands inside the container, use:
+
+```sh
+apptainer exec python_3.10.sif python -c "print('Hello from container')"
+```
+
+This runs a Python command inside the containerized environment.
+
+## Step 4: Run a Simple Scientific Example
+
+Containers are commonly used in research workflows. For example, you can run a simple NumPy computation:
+
+```sh
+apptainer exec python_3.10.sif python -c "import numpy as np; print(np.arange(5))"
+```
+
+This demonstrates how containerized environments can be used to run scientific Python code without installing dependencies locally.
+
+## Step 5: Explore the Container Environment
+
+Start an interactive shell inside the container:
+
+```sh
+apptainer shell python_3.10.sif
+```
+
+Once inside, you can run commands as if you were in a separate environment:
+
+```sh
+python
+```
+
+To exit the container:
+
+```sh
+exit
+```