COPY instruction for adding files in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the result of this Dockerfile COPY command?

Given this Dockerfile snippet:

FROM alpine
COPY ./app /usr/src/app

What happens when you build this Docker image?

Docker

FROM alpine
COPY ./app /usr/src/app

A The image build fails because the source path is missing a trailing slash.

B The file 'app' is copied as '/usr/src/app' inside the image.

C The image build ignores the COPY command and continues.

D The local folder 'app' is copied into the image at '/usr/src/app'.

💡 Hint

COPY copies files or folders from your computer into the image.

COPY copies the local folder named 'app' into the image at the specified path. The source path './app' is a folder, so the entire folder is copied.

🧠 Conceptual

intermediate

Understanding relative paths in COPY instruction

In a Dockerfile, what does the source path in COPY refer to?

A The path relative to the root directory of the host machine.

B The absolute path inside the container filesystem.

C The path relative to the Docker build context directory.

D The path relative to the Dockerfile location.

💡 Hint

Think about where Docker looks for files when building.

The source path in COPY is relative to the build context, which is the folder you specify when running 'docker build'.

❓ Configuration

advanced

Which COPY command correctly copies multiple files?

You want to copy two files, 'index.html' and 'style.css', from your local folder into '/usr/share/nginx/html' inside the image. Which COPY command is correct?

A COPY index.html style.css /usr/share/nginx/html/

B COPY ["index.html", "style.css", "/usr/share/nginx/html/"]

C COPY index.html /usr/share/nginx/html/ style.css

D COPY /usr/share/nginx/html/ index.html style.css

💡 Hint

COPY can take multiple source files followed by one destination folder.

Options A and B correctly copy multiple source files to the destination directory (A uses space-separated syntax, B uses JSON array form). Options C and D have wrong order or paths.

❓ Troubleshoot

advanced

Why does this COPY command fail during build?

Dockerfile snippet:

FROM ubuntu
COPY ./config /etc/config

When building, you get an error: 'COPY failed: stat /var/lib/docker/tmp/...: no such file or directory'. What is the likely cause?

Docker

FROM ubuntu
COPY ./config /etc/config

A The 'config' folder does not exist in the build context directory.

B Docker daemon does not have permission to write to '/etc/config'.

C The Dockerfile syntax is incorrect; COPY needs a trailing slash on source.

D The destination path '/etc/config' is invalid inside the container.

💡 Hint

Check if the source folder is present where Docker expects it.

The error means Docker cannot find the source folder in the build context. The destination path inside the container is valid and permissions are not checked at build time.

✅ Best Practice

expert

What is the best practice for copying files to reduce Docker image size?

You want to copy only necessary files into your Docker image to keep it small. Which COPY instruction approach is best?

A Use .dockerignore to exclude unnecessary files and COPY only needed files.

B COPY the entire project folder to avoid missing files.

C COPY files one by one to control what is added.

D Use ADD instead of COPY to automatically extract archives.

💡 Hint

Think about how to prevent unwanted files from being sent to Docker build.

Using .dockerignore excludes files from the build context, so COPY only adds needed files, reducing image size. Copying entire folders or files one by one is less efficient. ADD has extra features but is not recommended for simple copying.