Exploring and Inspecting Images

Remember our blueprint analogy? A good architect can read a blueprint and understand the building without actually building it.

They can see:

• The floor plan (file system)
• The specifications (metadata)
• The materials list (layers and sizes)
• The construction history (how it was built)

Docker gives you tools to “read the blueprint” of any image — to understand its structure, contents, and history without running a container.

Listing Local Images

docker images
# or
docker image ls

Shows all locally stored images with repository, tag, ID, creation date, and size.

Inspecting Image Details

docker image inspect nginx:latest

Returns a JSON document with everything Docker knows about the image:

• Configuration (environment variables, exposed ports, entry point)
• File system layers
• Creation date and author
• Architecture (amd64, arm64)

Viewing Image History

docker history nginx:latest

Shows how the image was built — layer by layer. Each line is a layer, showing:

• What command created it (if visible)
• How big it is
• When it was created

Checking Image Size

docker images --format "{{.Repository}}:{{.Tag}} {{.Size}}"

Shows just names and sizes for quick comparison.

When Inspecting an Image:

1. Base Image: What’s the underlying OS?

   • Debian/Ubuntu: Larger, more compatible
   • Alpine: Tiny, but can have compatibility issues

2. Exposed Ports: What ports does the application expect?

   "ExposedPorts": {
       "80/tcp": {},
       "443/tcp": {}
   }
   

3. Environment Variables: What can you configure?

   "Env": [
       "PATH=/usr/local/sbin:/usr/local/bin...",
       "NGINX_VERSION=1.24.0"
   ]
   

4. Entry Point and Command: What runs by default?

   "Entrypoint": ["docker-entrypoint.sh"],
   "Cmd": ["nginx", "-g", "daemon off;"]
   

5. Working Directory: Where does execution happen?

   "WorkingDir": "/app"
   

When Viewing History:

1. Layer Count: More layers = more complexity, potentially larger
2. Large Layers: Which steps add the most size?
3. Squashed Layers: <missing> in history means layers were squashed

Looking Inside an Image (Without Running)

You can create a container without starting it, then explore its file system:

docker create --name temp nginx:latest
docker export temp | tar -tvf - | head -50
docker rm temp

Using Dive Tool

Dive is a tool for exploring image layers:

dive nginx:latest

It shows an interactive view of:

• Each layer and its contents
• What files each layer adds/modifies
• Wasted space and efficiency metrics

---