Linux as a Virtualization Guest Containers

Hello — this lesson demonstrates practical container management and configuration using Docker-compatible tooling (Podman on RHEL-based systems). We’ll cover why containers are useful, how to install and configure Podman, and basic workflows: pulling images, running containers, mapping ports, stopping/removing containers, and cleaning up images.

Why use containers?

Containers package an application and everything it needs (binaries, libraries, configuration, logs, and data) together. This eliminates fragmented configurations across /etc, /var/lib, /var/log, etc., and makes migration or replication to other hosts predictable and repeatable.
Example: A MariaDB server inside a container keeps the daemon, configuration files, databases, and logs together. Copy or pull that container on another host and it behaves the same way.

Installing Docker-compatible tooling

On modern RHEL-based distributions (CentOS Stream, RHEL) Docker packages may not be available; Podman is the recommended replacement. Podman is daemonless, OCI-compliant, and provides Docker-compatible CLI behavior via the podman-docker wrapper.

Install Podman with dnf:

sudo dnf install podman

The podman-docker package provides docker CLI compatibility so many existing Docker commands (docker pull, docker run, docker ps, etc.) work unchanged.

Podman includes a Docker-CLI compatibility wrapper (podman-docker). You can use docker ... commands, or call podman directly. Podman is daemonless and integrates with systemd and rootless workflows.

Configuring default registries (optional)

Container tools consult /etc/containers/registries.conf to determine search registries for unqualified image names. To prefer docker.io only, set the unqualified-search-registries array to [“docker.io”].

Example excerpt from /etc/containers/registries.conf:

# An array of host[:port] registries to try when pulling an unqualified image, in order.
unqualified-search-registries = ["docker.io"]

Suppress the informational docker wrapper message by creating the marker file:

sudo touch /etc/containers/no-docker

Working with images and containers (nginx walkthrough) Below is a practical workflow for finding, pulling, running, testing, and cleaning up an nginx container using the Docker-compatible CLI. Replace docker with podman if you prefer explicit Podman commands. Search for nginx images in the registry:

docker search nginx

Pull the official nginx image (long and short forms both work):

docker pull docker.io/library/nginx
# or
docker pull nginx

Example pull output (truncated):

Trying to pull docker.io/library/nginx:latest...
Getting image source signatures
Copying blob c4ffe9532b5f done
Copying blob 2215908dc0a2 done
Copying config 12766a6745 done
Writing manifest to image destination
Storing signatures
12766a6745eea133de9fdcd03ff720fa971fdaf2
[aaron@LFCS-CentOS ~]$

List images available locally:

docker images

Example:

REPOSITORY                 TAG       IMAGE ID       CREATED         SIZE
docker.io/library/nginx    latest    12766a6745ee   17 hours ago    146 MB
docker.io/library/nginx    1.20.2    8f34c303855f   17 hours ago    146 MB

Remove an older image tag:

docker rmi nginx:1.20.2

Example output:

Untagged: docker.io/library/nginx:1.20.2
Deleted: 8f34c303855f0e55ca018386ff45cf47b1d7da554ab746b1c60049ebcf12d42c
[aaron@LFCS-CentOS ~]$

Creating and running containers

By default, running without -d attaches your terminal to the container’s stdout/stderr. Use Ctrl+C to stop.

Run in the foreground (attached):

docker run nginx

Run detached (background):

docker run -d nginx
# outputs container ID, e.g.:
# 92a87f978de3...

List running containers:

docker ps

List all containers (running and stopped):

docker ps --all

Stop and remove containers:

docker stop <container-name-or-id>
docker rm <container-name-or-id>

Notes on safe image removal:

docker rmi fails if an image is in use by any container. Preferred safe removal sequence:
1. Stop containers: docker stop <container>
2. Remove containers: docker rm <container>
3. Remove image: docker rmi <image>

Naming containers and exposing ports

Assign a name and publish ports with —name and -p HOST:CONTAINER. Example: run nginx named mywebserver with host port 8080 mapped to container port 80:

docker run -d -p 8080:80 --name mywebserver nginx
# Example returned container ID:
# 7953475436b9...

Testing the web server with netcat

Use netcat (nc) to query the server. Pass host and port as separate arguments (no colon):

nc localhost 8080
GET /

You should receive the nginx default index.html content:

<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...
</html>

Press Ctrl+C to exit nc.

Ports below 1024 are privileged. To bind host port 80 to a container, run the command with elevated privileges (sudo) or map a non-privileged host port (>=1024).

Binding to privileged port 80 (example):

sudo docker run -d -p 80:80 --name mywebserver nginx

Getting help and Podman-specific options

Get detailed help for any command with —help:

docker run --help
docker rm --help
# or
podman run --help

Podman rm options (example excerpt):

podman rm [options] CONTAINER [CONTAINER...]

Examples:
  podman rm imageID
  podman rm mywebserver myflaskserver 860a4b23
  podman rm --force --all
  podman rm -f c684f0d469f2

Options:
  -a, --all           Remove all containers
  -f, --force         Force removal of a running or unusable container
  -v, --volumes       Remove anonymous volumes associated with the container
  -t, --time uint     Seconds to wait for stop before killing the container (default 10)
  ...

Quick reference: common Docker/Podman commands

Action	Command
Search registry	`docker search <term>`
Pull image	`docker pull \<image>`
List local images	`docker images`
Run container (foreground)	`docker run \<image>`
Run detached	`docker run -d -p <host>:\<container> --name <name> \<image>`
List running containers	`docker ps`
List all containers	`docker ps --all`
Stop container	`docker stop \<container>`
Remove container	`docker rm \<container>`
Remove image	`docker rmi \<image>`

Links and references

This concludes the short hands-on introduction to pulling images, running containers, mapping ports, and cleaning up images. Practice these commands in a lab environment to build confidence and reinforce container lifecycle management.

Introduction

System Architecture

Linux Installation and Package Management

GNU and Unix Commands

Devices, Linux Filesystems, Filesystem Hierarchy Standard

Conclusion and Mock Exam

Linux as a Virtualization Guest Containers

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