Cache mount for faster builds in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker build with cache mount

What will be the output of the following Docker build command snippet when using cache mount for pip packages?

Docker

RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt

A Docker build fails with syntax error due to invalid mount option.

B Docker ignores the cache mount and installs packages from scratch every time.

C Docker uses the cache directory to speed up pip installs, reducing build time.

D Docker caches the entire image layer, not just pip cache, causing larger image size.

💡 Hint

Think about what the cache mount does for package managers during builds.

The --mount=type=cache option tells Docker to save and reuse the pip cache directory between builds. This speeds up package installation by avoiding repeated downloads.

🧠 Conceptual

intermediate

Purpose of cache mounts in Docker builds

Why are cache mounts used in Docker builds?

A To persist data between build steps and speed up repeated operations like package installs.

B To increase the size of the Docker image for better caching.

C To share files between running containers at runtime.

D To permanently store build artifacts inside the final image.

💡 Hint

Think about what slows down builds and how cache helps.

Cache mounts allow Docker to save data like package caches between builds, so repeated operations don't redo work, making builds faster.

❓ Troubleshoot

advanced

Troubleshooting cache mount syntax error

You run this Dockerfile line but get a syntax error: RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt. What is the likely cause?

A Cache mounts are not supported on Linux hosts.

B The cache mount target path is invalid and must be an absolute path inside /tmp.

C The pip install command is incorrect and causes the error.

D The Dockerfile is missing the syntax directive: <code># syntax=docker/dockerfile:1.4</code>.

💡 Hint

Check Dockerfile version and syntax requirements for cache mounts.

Cache mounts require Dockerfile syntax version 1.4 or higher, specified by # syntax=docker/dockerfile:1.4 at the top of the Dockerfile.

🔀 Workflow

advanced

Optimizing Docker build with cache mounts

Which Dockerfile snippet best uses cache mounts to speed up npm package installation during build?

A RUN --mount=type=cache,target=/root/.npm npm install

B RUN npm install --cache /root/.npm

C RUN --mount=type=cache,target=/root/.cache/npm npm install

D RUN --mount=type=bind,target=/root/.cache/npm npm install

💡 Hint

Consider the correct cache directory for npm and mount type.

npm stores cache in /root/.npm. Using type=cache mount on this directory speeds up installs by caching downloaded packages.

✅ Best Practice

expert

Best practice for cache mount usage in multi-stage builds

In a multi-stage Docker build, where should you place cache mounts for package managers to maximize build speed and minimize final image size?

A Use cache mounts in every stage to ensure all layers cache package data.

B Use cache mounts only in the build stage where packages are installed, not in the final stage.

C Use cache mounts only in the final stage to keep cache data in the final image.

D Avoid cache mounts in multi-stage builds to prevent cache pollution.

💡 Hint

Think about where package installation happens and what ends up in the final image.

Cache mounts should be used in the build stage to speed up package installs. The final stage should be clean without cache data to keep image size small.