Container Intro

Containers refer to a lightweight, portable, and self-sufficient unit of software that puts together code and all its dependencies so one can run an application/service.

Virtual machine (VM)s virtualize the underlying hardware so that multiple operating system (OS) instances can run on that hardware. Each VM runs an OS and has access to virtualized resources representing the underlying hardware. On the other hand, a container virtualizes the underlying OS and causes the containerized app to perceive that it has the OS—including CPU, memory, file storage, and network connections—all to itself. Besides, containers share the host OS, so they do not need to boot an OS. So contenarized applications can start much faster.

There are several container runtimes available, including Docker, Podman, containerd, Apptainer (Singularity).

Quick docker example

Here we assume you have docker install on your machine (e.g. on macos: brew install --cask docker or install it by going to the docker website).

Once installed you can run

$ docker run hello-world
Hello from Docker!
This message shows that your installation appears to be working correctly.

$ docker images
REPOSITORY    TAG       IMAGE ID       CREATED        SIZE
hello-world   latest    74cc54e27dc4   4 months ago   10.1kB

You can create your own image. (files: folder) )

Create app.py with this content:

# app.py
print("Hello, world from Docker!")

Create this Dockerfile:

Dockerfile

# Use official Python image
FROM python:3.12-slim

# Set working directory
WORKDIR /app

# Copy source code
COPY app.py .

# Run the script
CMD ["python", "app.py"]

Run:

$ docker build -t hello-python .
$ docker run hello-python
Hello, world from Docker!

$ docker images
REPOSITORY     TAG       IMAGE ID       CREATED          SIZE
hello-python   latest    e8cc71bba03c   45 seconds ago   124MB
hello-world    latest    74cc54e27dc4   4 months ago     10.1kB

Quick podman example

Podman has basically the same syntax then docker. However, docker does not have a daemon as a requirement.

Install it:

brew install podman

Start the VM (required in macos; for linux you don't need that)

podman machine init
podman machine start

Then you can do the same commands as described in the docker section above. For instance:

$ podman run hello-world
Resolved "hello-world" as an alias (/etc/containers/registries.conf.d/000-shortnames.conf)
Trying to pull quay.io/podman/hello:latest...
Getting image source signatures
Copying blob sha256:81df7ff16254ed9756e27c8de9ceb02a9568228fccadbf080f41cc5eb5118a44
Copying config sha256:5dd467fce50b56951185da365b5feee75409968cbab5767b9b59e325fb2ecbc0
Writing manifest to image destination
!... Hello Podman World ...!

You can check the podman VM running:

$ podman machine list
NAME                     VM TYPE     CREATED        LAST UP            CPUS        MEMORY      DISK SIZE
podman-machine-default*  applehv     4 minutes ago  Currently running  4           2GiB        100GiB

You can turn it off:

podman machine stop

Other notes

If you want to get into a container image to explore file system for instance:

docker run -it --rm <myimage> bash

-it: interactive terminal
--rm: remove container on exit
<myimage>: use the image your original container used

You can check the content of a file in the image for instance:

docker run -it --rm hello-python cat /app/app.py

If you want to get into a running container:

docker exec -it <mycontainer> bash

Or just check something there:

docker exec -it <mycontainer> ls /app

Linux

In Linux, docker may fail because it says the daemon: "docker: permission denied while trying to connect to the Docker daemon socket"

By default, Docker runs as a root-owned service. To manage Docker as a non-root user, your user needs to be part of the docker group. This allows you to run Docker commands without needing sudo. Add user to docker group:

sudo usermod -aG docker $USER