Containers, Docker, and Podman – Sébastien De Revière

What is a Container?

A container is an instance of an image, which is a lightweight, standalone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.

Consistency across environments

Regardless of where you’re running your application - locally, in a testing environment, or in production - it will work the same way inside a container.

Isolation

Each container runs in isolation from other containers, ensuring that they don’t interfere with each other. This is similar to virtual machines (VMs), but with an important difference. While VMs virtualize the hardware, providing a full operating system stack for each VM, containers share the same host system kernel and isolate the application processes. This makes containers much lighter and faster than VMs.

Portability

You can run your containerized application on any system that supports containerization technology, without worrying about system-specific dependencies.

Docker vs Podman: A Detailed Comparison

While Docker has been the go-to container platform for a long time, Podman offers some unique advantages.

Daemonless

Unlike Docker, which relies on a daemon, Podman doesn’t require a daemon to function, leading to improved security and less overhead.

Rootless Containers

Podman allows you to run containers without root privileges, providing additional security.

Open Container Initiative (OCI) Compliance

Both Docker and Podman are compliant with the OCI specifications. However, Podman’s implementation is stricter and does not include Docker’s non-standard features.

Command Compatibility

Podman is completely command-compatible with Docker, meaning you can use the same commands and Dockerfiles as you would with Docker.

Leveraging Containers for Efficient Workflow

Development Efficiency

For developers, containers guarantee that the environment remains consistent across development, testing, and production stages. This means no more “It works on my machine” issues.

Simplified Testing

If your role involves software testing, you can quickly spin up an isolated environment with containers to test different versions of your application under varying conditions.

Smoothened Deployment

Deploying software with containers ensures that the software behaves exactly as intended, regardless of the deployment environment.

Podman Commands (compatible with Docker)

Podman commands are identical to Docker commands, ensuring a smooth transition between the two. Here are some of the key Podman commands you’ll use:

Pull an Image

podman pull registry_name/repository_name:image_tag

The :image_tag is optional and defaults to :latest if not provided.

Build an Image

podman build -t my_image .

Run a Container

podman run -d --name my_container my_image

Run a Container Interactively with a Custom Hostname and Name, and Remove it after Exit

podman run -it --rm --hostname=my_hostname --name=my_container my_image

List Running Containers

podman ps

List all Containers, Including Stopped Ones

podman ps -a

Start a Container and Attach its Output

podman start -a my_container

View Logs for a Container

podman logs my_container

Execute a Command Inside a Running Container

podman exec -it my_container command

Create a new image from a container’s changes

podman commit -c 'USER your_username' -c 'WORKDIR /path/to/workdir' my_container my_new_image

The -c or (--change) option allows to apply Dockerfile instructions to the image we are creating.

Initialize a Container

podman init my_container

Stop a Container

podman stop my_container

Remove a Container

podman rm my_container

List Images

podman images

Remove an Image

podman rmi my_image