How to Use Dockerfiles with Podman on AlmaLinux

Podman is an increasingly popular alternative to Docker for managing containers, and it is fully compatible with OCI (Open Container Initiative) standards. If you’re running AlmaLinux, a community-supported, enterprise-grade Linux distribution, you can leverage Podman to build, manage, and deploy containers efficiently using Dockerfiles. In this blog post, we’ll dive into the steps to use Dockerfiles with Podman on AlmaLinux.

Introduction to Podman and AlmaLinux

Podman is a container management tool that provides a seamless alternative to Docker. It offers daemonless and rootless operation, which enhances security by running containers without requiring root privileges. AlmaLinux, an enterprise-ready Linux distribution, is a perfect host for Podman due to its stability and compatibility with RHEL ecosystems.

When using Podman on AlmaLinux, Dockerfiles are your go-to tool for automating container image creation. They define the necessary steps to build an image, allowing you to replicate environments and workflows efficiently.

Understanding Dockerfiles

A Dockerfile is a text file containing instructions to automate the process of creating a container image. Each line in the Dockerfile represents a step in the build process. Here’s an example:

# Use an official base image
FROM ubuntu:20.04

# Install dependencies
RUN apt-get update && apt-get install -y curl

# Add a file to the container
COPY myapp /usr/src/myapp

# Set the working directory
WORKDIR /usr/src/myapp

# Define the command to run
CMD ["./start.sh"]

The Dockerfile is the foundation for creating customized container images tailored to specific applications.

Prerequisites

Before proceeding, ensure you have the following:

1. AlmaLinux Installed: A working installation of AlmaLinux with a non-root user having sudo privileges.
2. Podman Installed: Installed and configured Podman (steps below).
3. Basic Dockerfile Knowledge: Familiarity with Dockerfile syntax is helpful but not required.

Installing Podman on AlmaLinux

To start using Dockerfiles with Podman, you must install Podman on your AlmaLinux system.

Step 1: Update the System

Update your package manager to ensure you have the latest software versions:

sudo dnf update -y

Step 2: Install Podman

Install Podman using the default AlmaLinux repository:

sudo dnf install -y podman

Step 3: Verify the Installation

Check the installed version to ensure Podman is set up correctly:

podman --version

Creating a Dockerfile

Let’s create a Dockerfile to demonstrate building a simple image with Podman.

Step 1: Set Up a Workspace

Create a directory for your project:

mkdir ~/podman-dockerfile-demo
cd ~/podman-dockerfile-demo

Step 2: Write the Dockerfile

Create a Dockerfile in the project directory:

nano Dockerfile

Add the following content to the Dockerfile:

# Start with an official base image
FROM alpine:latest

# Install necessary tools
RUN apk add --no-cache curl

# Copy a script into the container
COPY test.sh /usr/local/bin/test.sh

# Grant execute permissions
RUN chmod +x /usr/local/bin/test.sh

# Set the default command
CMD ["test.sh"]

Step 3: Create the Script File

Create a script file named test.sh in the same directory:

nano test.sh

Add the following content:

#!/bin/sh
echo "Hello from Podman container!"

Make the script executable:

chmod +x test.sh

Building Images Using Podman

Once the Dockerfile is ready, you can use Podman to build the image.

Step 1: Build the Image

Run the following command to build the image:

podman build -t my-podman-image .

• -t my-podman-image: Tags the image with the name my-podman-image.
• .: Specifies the current directory as the context.

You’ll see output logs as Podman processes each instruction in the Dockerfile.

Step 2: Verify the Image

After the build completes, list the available images:

podman images

The output will show the new image my-podman-image along with its size and creation time.

Running Containers from the Image

Now that the image is built, you can use it to run containers.

Step 1: Run the Container

Run a container using the newly created image:

podman run --rm my-podman-image

The --rm flag removes the container after it stops. The output should display:

Hello from Podman container!

Step 2: Run in Detached Mode

To keep the container running in the background, use:

podman run -d --name my-running-container my-podman-image

Verify that the container is running:

podman ps

Managing and Inspecting Images and Containers

Listing Images

To see all locally available images, use:

podman images

Inspecting an Image

To view detailed metadata about an image, run:

podman inspect my-podman-image

Stopping and Removing Containers

Stop a running container:

podman stop my-running-container

Remove a container:

podman rm my-running-container

Troubleshooting Common Issues

1. Error: Permission Denied

If you encounter a “permission denied” error, ensure you’re running Podman in rootless mode and have the necessary permissions:

sudo usermod -aG podman $USER
newgrp podman

2. Build Fails Due to Network Issues

Check your network connection and ensure you can reach the Docker registry. If using a proxy, configure Podman to work with it by setting the http_proxy environment variable.

3. SELinux Denials

If SELinux blocks access, inspect logs for details:

sudo ausearch -m avc -ts recent

Temporarily set SELinux to permissive mode for debugging:

sudo setenforce 0

Conclusion

Using Dockerfiles with Podman on AlmaLinux is an efficient way to build and manage container images. This guide has shown you how to create a Dockerfile, build an image with Podman, and run containers from that image. With Podman’s compatibility with Dockerfile syntax and AlmaLinux’s enterprise-grade stability, you have a powerful platform for containerization.

By mastering these steps, you’ll be well-equipped to streamline your workflows, automate container deployments, and take full advantage of Podman’s capabilities. Whether you’re new to containers or transitioning from Docker, Podman offers a secure and flexible environment for modern development.

Let us know about your experiences with Podman and AlmaLinux in the comments below!