Using .dockerignore - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Effect of .dockerignore on Docker build context size

You have a project folder with 100 files. Your .dockerignore file contains *.log and temp/. Which files will NOT be sent to the Docker daemon during docker build?

A All files including .log files and temp directory files

B All files except those ending with .log and all files inside the temp directory

C Only files ending with .log and files inside temp directory

D Only files outside the project folder

💡 Hint

Think about what *.log and temp/ patterns exclude.

The .dockerignore file excludes files matching its patterns from the build context. Here, files ending with .log and everything inside the temp folder are excluded and not sent to Docker daemon.

🧠 Conceptual

intermediate

Purpose of .dockerignore in Docker builds

Why is it important to use a .dockerignore file in your Docker project?

A To define the order of Dockerfile instructions

B To specify which Docker images to pull before building

C To list environment variables for the container

D To reduce the size of the build context sent to the Docker daemon, speeding up builds and avoiding unnecessary files

💡 Hint

Think about what happens when Docker sends files to build the image.

The .dockerignore file tells Docker which files to ignore when sending the build context. This reduces build time and avoids including sensitive or unnecessary files.

❓ Troubleshoot

advanced

Why is a large file still included despite .dockerignore?

You added secret.txt to your .dockerignore file, but it still appears in the Docker image after build. What is the most likely reason?

A The <code>secret.txt</code> file was copied in a previous build layer and cached, so the cache must be cleared

B Docker does not support ignoring individual files

C The <code>.dockerignore</code> file must be named <code>dockerignore.txt</code>

D The Dockerfile must include <code>IGNORE secret.txt</code> instruction

💡 Hint

Think about Docker build cache and layers.

If a file was included in a previous build layer, Docker may reuse that cached layer. Even if .dockerignore excludes it now, the cache can cause the file to appear. Clearing the cache or forcing a rebuild fixes this.

✅ Best Practice

advanced

Best practice for .dockerignore patterns

Which of the following .dockerignore patterns correctly excludes all files in the node_modules folder and any .env files anywhere in the project?

A

node_modules
.env

B

node_modules/
**/.env

C

/node_modules
.env

D

node_modules/*
.env/*

💡 Hint

Remember that / at the end means folder, and **/ matches files anywhere.

node_modules/ excludes the entire folder and its contents. **/.env matches any .env file in any subfolder. Other options either miss the folder or do not match files correctly.

🔀 Workflow

expert

Order of operations when building with .dockerignore

Put these steps in the correct order when building a Docker image with a .dockerignore file:

A 4,1,2,3

B 1,4,2,3

C 4,2,1,3

D 1,2,4,3

💡 Hint

Think about what happens first when you run docker build.

First, the Docker client starts the build (4). Then it reads .dockerignore to exclude files (1). Next, it sends the filtered context to the daemon (2). Finally, the daemon runs the Dockerfile instructions (3).