Analyzing image layers with dive in Docker - Commands & Configuration

Introduction

When you build Docker images, they are made of layers. Sometimes these layers get too big or inefficient. Dive helps you look inside these layers to see what files are added or changed, so you can make your images smaller and faster.

When you want to find out why your Docker image is very large.

When you want to see which files are added in each layer of your Docker image.

When you want to optimize your Dockerfile to reduce image size.

When you want to check if unnecessary files are included in your image.

When you want to understand how your Docker image is built step-by-step.

Commands

This command builds a Docker image named 'example-app' using the Dockerfile in the current folder. We do this first to have an image to analyze with dive.

Terminal

docker build -t example-app:latest .

Expected Output

Sending build context to Docker daemon  3.07kB

Step 1/3 : FROM alpine:3.18

3.18: Pulling from library/alpine

Digest: sha256:...

Status: Downloaded newer image for alpine:3.18

 ---> 3f53bb00af94

Step 2/3 : COPY app.sh /app.sh

 ---> Using cache

 ---> 8c7f9a1f9e2a

Step 3/3 : CMD ["/app.sh"]

 ---> Running in 4d5a6b7c8d9e

Removing intermediate container 4d5a6b7c8d9e

 ---> 1a2b3c4d5e6f

Successfully built 1a2b3c4d5e6f

Successfully tagged example-app:latest

→

-t - Names and tags the image for easy reference.

This command opens dive to analyze the 'example-app:latest' image. Dive shows each layer, what files it adds or changes, and the size impact.

Terminal

dive example-app:latest

Expected Output

Launching dive UI...

[UI opens showing image layers, file changes, and size details]

Key Concept

If you remember nothing else from this pattern, remember: dive lets you see inside Docker image layers to find and fix size and efficiency problems.

Common Mistakes

Trying to run dive without building the Docker image first.

Dive needs an existing image to analyze; without it, dive will show an error or no data.

Always build your Docker image with 'docker build' before running dive.

Not tagging the image during build and then using the wrong image name in dive.

Dive cannot find the image if the name or tag is incorrect.

Use the exact image name and tag you set with 'docker build -t' when running dive.

Summary

Build your Docker image with 'docker build -t example-app:latest .' to create an image to analyze.

Run 'dive example-app:latest' to open a visual tool showing each image layer and its contents.

Use dive to find large or unnecessary files in layers and improve your Dockerfile for smaller images.

Practice

1. What is the main purpose of using dive with Docker images?

easy

A. To visually explore and analyze Docker image layers

B. To build Docker images faster

C. To deploy Docker containers automatically

D. To monitor running Docker containers

Solution

1. Step 1: Understand what dive does

   dive is a tool designed to help users explore Docker image layers visually and understand their contents.

2. Step 2: Compare options with dive's purpose

   Only To visually explore and analyze Docker image layers describes the visual exploration and analysis of image layers, which matches dive's main use.

3. Final Answer:

   To visually explore and analyze Docker image layers -> Option A

4. Quick Check:

   Purpose of dive = Visual analysis [OK]

Hint: Remember dive = visual layer explorer for Docker images [OK]

Common Mistakes:

• Confusing dive with image build tools
• Thinking dive manages containers
• Assuming dive deploys images

2. Which command correctly starts analyzing a Docker image named myapp with tag v1 using dive?

easy

A. dive --analyze myapp v1

B. dive -image myapp v1

C. dive myapp:v1

D. dive run myapp:v1

Solution

1. Step 1: Recall dive command syntax

   The correct syntax to analyze an image is dive <image-name>:<tag>.

2. Step 2: Match options to syntax

   Only dive myapp:v1 matches the correct syntax exactly: dive myapp:v1.

3. Final Answer:

   dive myapp:v1 -> Option C

4. Quick Check:

   Correct dive syntax = dive myapp:v1 [OK]

Hint: Use dive with image:tag format, no extra flags needed [OK]

Common Mistakes:

• Adding unsupported flags like --analyze
• Separating image and tag with space
• Using 'run' keyword with dive

3. After running dive nginx:latest, you see a layer with a large size but few added files. What does this likely indicate?

medium

A. The image is corrupted

B. The layer added many new files

C. The layer is empty and can be removed

D. The layer contains mostly metadata or deleted files

Solution

1. Step 1: Interpret layer size vs file count

   A large layer size with few added files often means the layer contains metadata changes or deleted files that still occupy space.

2. Step 2: Eliminate other options

   The layer added many new files contradicts the few files seen. The image is corrupted is unlikely without error messages. The layer is empty and can be removed is false because the layer is not empty.

3. Final Answer:

   The layer contains mostly metadata or deleted files -> Option D

4. Quick Check:

   Large size + few files = metadata/deleted files [OK]

Hint: Large layer but few files? Check for metadata or deletions [OK]

Common Mistakes:

• Assuming large size means many files
• Thinking the image is broken without evidence
• Believing empty layers can be removed safely

4. You ran dive myimage:latest but see an error: image not found locally. What is the best fix?

medium

A. Pull the image first using docker pull myimage:latest

B. Run dive with --force flag

C. Rebuild the image with docker build

D. Restart the Docker daemon

Solution

1. Step 1: Understand the error meaning

   The error means dive cannot find the image locally to analyze it.

2. Step 2: Choose the correct fix

   Pulling the image locally with docker pull myimage:latest makes it available for dive.

3. Final Answer:

   Pull the image first using docker pull myimage:latest -> Option A

4. Quick Check:

   Image not found = pull image first [OK]

Hint: Pull missing images before analyzing with dive [OK]

Common Mistakes:

• Using unsupported dive flags
• Rebuilding image unnecessarily
• Restarting Docker without cause

5. You want to reduce your Docker image size by analyzing layers with dive. Which approach is best after identifying large layers with redundant files?

hard

A. Increase container memory to handle large layers

B. Rewrite the Dockerfile to combine commands and remove unnecessary files before each layer

C. Use dive's built-in optimizer to auto-shrink the image

D. Delete layers manually from the image using dive

Solution

1. Step 1: Understand dive's role

   dive helps identify large or redundant layers but does not modify images directly.

2. Step 2: Choose the best optimization method

   Optimizing the Dockerfile by combining commands and cleaning files reduces layer size effectively.

3. Step 3: Eliminate incorrect options

   Delete layers manually from the image using dive is impossible; dive cannot delete layers. Use dive's built-in optimizer to auto-shrink the image is false; dive has no auto-optimizer. Increase container memory to handle large layers is unrelated to image size.

4. Final Answer:

   Rewrite the Dockerfile to combine commands and remove unnecessary files before each layer -> Option B

5. Quick Check:

   Optimize Dockerfile after dive analysis = Rewrite the Dockerfile to combine commands and remove unnecessary files before each layer [OK]

Hint: Fix Dockerfile to shrink layers after dive analysis [OK]

Common Mistakes:

• Expecting dive to auto-fix images
• Trying to delete layers manually
• Confusing container memory with image size