DevOps Day 39: Creating a Docker Image from a Running Container¶

Today's task was a practical exercise in a very useful, though less common, method of creating Docker images. Instead of writing a Dockerfile, my objective was to create a new image by "snapshotting" the current state of a container that was already running. This is a powerful technique for debugging and capturing a specific, manually configured state.

I learned how to use the docker commit command to save a container's changes as a new, reusable image. This document is my first-person guide to that process, explaining the concepts and the commands I used.

The Task¶

My objective was to create a new Docker image on App Server 1. The specific requirements were: - The new image must be named news:devops. - It had to be created from the current state of a running container named ubuntu_latest.

My Step-by-Step Solution¶

Connect to the Server: I first logged into App Server 1 (ssh tony@stapp01).
Verify the Source Container: As a best practice, I confirmed that the source container was running using sudo docker ps. This showed the ubuntu_latest container with an "Up" status.
Create the Image from the Container: This was the core of the task. I used a single docker commit command.
```
sudo docker commit ubuntu_latest news:devops
```
The command returned the unique ID of the new image, which was my first sign of success.
Verification: The crucial final step was to confirm that my new image existed. I used the docker images command.
```
sudo docker images
```
The output clearly showed my new news:devops image at the top of the list, which was the definitive proof that my task was completed successfully.

Key Concepts (The "What & Why")¶

docker commit: This is the key command for this task. It takes a container's current state—including any changes made to its filesystem after it was started (like new files, installed packages, or configuration changes)—and creates a new image from it. It's essentially a "snapshot" of the container's writable layer.
When to use docker commit: While building images with a Dockerfile is the standard for production because it's reproducible, docker commit is incredibly useful in specific scenarios:
1. Debugging: If a container is having issues, you can use docker exec to get inside, install debugging tools (like vim, curl), and then commit that container to a new myapp:debug image. This image can then be shared with other developers to analyze the problem in the exact same environment.
2. Saving a Manual State: Sometimes you might configure an application inside a container through a series of manual steps. Committing the result saves that specific state as a reusable image.
Dockerfile vs. docker commit: It's important to know the difference.
- Dockerfile is declarative and reproducible. It's a recipe that anyone can follow to get the same result. This is the professional standard for production images.
- docker commit is imperative and creates a "black box" image. No one knows how the container got into its current state unless you document it elsewhere. It's best used for temporary, ad-hoc situations like debugging.

Commands I Used¶

sudo docker ps: Printer Status. Lists all running Docker containers, which I used to verify my source container existed.
sudo docker commit ubuntu_latest news:devops: The main command for the task. It takes a snapshot of the ubuntu_latest container and saves it as a new image named news:devops. The syntax is docker commit <CONTAINER_NAME> <NEW_IMAGE_NAME:TAG>.
sudo docker images: Lists all the Docker images stored on the local server. I used this to verify that my new news:devops image was created successfully.