dev-ops-challenges

Pull Docker Image

Technical Overview

In Docker and containerized workflows, images are the read-only templates used to spin up container instances. They contain the application code, runtime libraries, dependencies, and environment configurations. Docker images are composed of multiple layers stacked on top of each other, where each layer represents an instruction in the image’s Dockerfile.

Docker Image Registries & Pulling

By default, the Docker Daemon is configured to pull images from Docker Hub, the industry-standard public container registry. When you issue a docker pull command, the Docker Daemon downloads each layer of the specified image tag asynchronously and stores them in the host’s local image cache.

Image Tagging and Aliasing

Docker tags are used to distinguish different versions, configurations, or builds of the same image (e.g., alpine, latest, musl).

Using the docker tag command, you can create an alias/new name pointing to an existing local image. This does not duplicate the image files or download new layers. Instead, it creates a new reference pointer pointing to the exact same Image ID (cryptographic hash of the image configuration).

sequenceDiagram
    participant Host as Docker Host (stapp01)
    participant Docker as Docker Daemon
    participant Hub as Docker Hub Registry

    Host->>Docker: Run `docker pull busybox:musl`
    Docker->>Hub: Request image layers for busybox:musl
    Hub-->>Docker: Stream image layers
    Docker-->>Host: Store image locally (Image ID: e.g., 6d84a7e)
    Host->>Docker: Run `docker tag busybox:musl busybox:media`
    Docker->>Docker: Create reference alias 'busybox:media' pointing to 6d84a7e
    Host->>Docker: Run `docker images`
    Docker-->>Host: Display both tags with identical Image ID (6d84a7e)

Infrastructure & Configuration Requirements

Target Host: Nautilus App Server 1 (stapp01) (can vary in labs, e.g., stapp01, stapp02, stapp03)
SSH User: tony (associated with stapp01; steve for stapp02, banner for stapp03)
Source Image to Pull: busybox:musl
Target Tag/Repository Name: busybox:media

Step-by-Step Implementation

Step 1: Connect to the Application Server

SSH from the Jump Host to App Server 1:

ssh tony@stapp01

Provide the server password when prompted.

Step 2: Pull the Busybox Image

Pull the busybox image with the specific musl tag from Docker Hub:

docker pull busybox:musl

Note: If permissions require, prepend with sudo:

sudo docker pull busybox:musl

Step 3: Verify the Image was Pulled

Confirm the image is cached locally and view its Image ID:

docker images

Expected Output:

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
busybox      musl      6d84a7e9e51c   2 weeks ago    1.45MB

Step 4: Retag the Image

Create an alias for the pulled image, tagging it as busybox:media:

docker tag busybox:musl busybox:media

Or with sudo if necessary:

sudo docker tag busybox:musl busybox:media

Post-Deployment Verification

1. Verify Both Tags Point to the Same Image ID

Run the list command again to verify that both tags exist and display the exact same IMAGE ID:

docker images

Expected Output:

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
busybox      media     6d84a7e9e51c   2 weeks ago    1.45MB
busybox      musl      6d84a7e9e51c   2 weeks ago    1.45MB

Note that the size and creation date are identical because they refer to the same physical layers on the host disk.

2. Inspect Image Details

To double-check the local repository metadata, run a docker inspect check:

docker inspect busybox:media

Log out of the Application Server:

exit