Understanding Images, Containers, and Registries

Before we start running FEniCS inside Docker, it’s important to understand three key terms you’ll see often — images, containers, and registries. Think of them as the building blocks of how Docker works.

Docker Images

A Docker image is like a blueprint or recipe. It defines everything your simulation needs — the operating system, Python, FEniCS, meshio, and any custom scripts.

You don’t run an image directly; instead, you use it to create a container (just like a class creates an object, if you’re familiar with programming).

Example:

• A FEniCS image might be called iitrabhi/fenics_notebook.
• It contains Ubuntu + FEniCS + Python + libraries.

You can download this image using:

docker pull iitrabhi/fenics_notebook

And once downloaded it should be visible in the docker images list:

Docker Containers

A container is a running instance of an image. It’s like having your own mini computer that runs inside your main system — with everything pre-configured.

You can create and start a container using:

docker run -it iitrabhi/fenics_notebook

Once inside, you’ll get a terminal where FEniCS is ready to use — no setup, no installations, just simulation.

When you stop the container, all changes stay inside it unless you save or export them.

Docker Registries

A registry is where Docker images are stored and shared — like a library for simulation environments.

The main public registry is Docker Hub (https://hub.docker.com), where you can find ready-to-use images such as:

• fenicsproject/stable
• paraview/paraview
• ubuntu

You can also create your own private registries to store internal project images.

In Simple Terms

Concept	Analogy	Purpose
Image	A recipe	Defines what goes inside your simulation environment
Container	A dish made from the recipe	A running environment where you actually execute code
Registry	A recipe library	A place to find and share images

Summary

In short:

• Images define your environment.
• Containers run your environment.
• Registries store and share them.

In the next section, we’ll use these concepts to run FEniCS inside a Docker container and see how simple it is to launch a ready-to-use simulation environment.