Create 04_docker_images.mdx

Author: Amanda-dong

docs/hpc/15_tutorial_apptainer/04_docker_images.mdx

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+# Using Docker Images with Apptainer
+
+So far, we have used container images that are already available on Torch under `/share/apps/images`.
+
+In practice, you may also want to run software that is not provided by the cluster. Apptainer can pull images directly from Docker registries and convert them into the Apptainer SIF format.
+
+For example, we can pull an official PyTorch image from Docker Hub:
+
+```bash
+apptainer pull pytorch.sif docker://pytorch/pytorch:latest
+```
+
+During the pull process, Apptainer downloads the Docker image layers and converts them into a single SIF image:
+
+```text
+INFO:    Converting OCI blobs to SIF format
+INFO:    Starting build...
+INFO:    Fetching OCI image...
+...
+INFO:    Creating SIF file...
+```
+
+The output shows that Apptainer is downloading the Docker image layers and converting them into a single SIF image. Once the conversion completes, the resulting SIF file can be used without Docker.
+When the command completes, a new image named `pytorch.sif` will be created in the current directory.
+
+You can verify that the image exists:
+
+```bash
+ls -lh pytorch.sif
+```
+
+The image can now be used like any other Apptainer image.
+
+For example:
+
+```bash
+apptainer exec pytorch.sif python --version
+```
+
+or
+
+```bash
+apptainer exec pytorch.sif python -c "import torch; print(torch.__version__)"
+```