Cache mount for faster builds in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to use a cache mount for faster builds.

Docker

RUN --mount=type=[1],target=/root/.cache pip install -r requirements.txt

Drag options to blanks, or click blank then click option'

Atmpfs

Bcache

Cbind

Dvolume

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'bind' instead of 'cache' causes no caching and slower builds.

Using 'tmpfs' mounts a temporary filesystem that is cleared after the build step.

The cache mount type allows Docker to cache files between builds, speeding up repeated steps like package installation.

2fill in blank

medium

Complete the Dockerfile RUN command to specify the cache mount with an ID.

Docker

RUN --mount=type=cache,[1]=mycache,target=/root/.cache pip install -r requirements.txt

Drag options to blanks, or click blank then click option'

Aid

Bkey

Csource

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' instead of 'id' causes Docker to ignore the cache.

Omitting the cache ID causes a new cache to be created every build.

The id option names the cache mount so Docker can reuse it across builds.

3fill in blank

hard

Fix the error in this Dockerfile RUN command to correctly use cache mount syntax.

Docker

RUN --mount=type=[1],target=/root/.cache,id=mycache pip install -r requirements.txt

Drag options to blanks, or click blank then click option'

Acache

Bbind

Ctmpfs

Dvolume

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'bind' or 'tmpfs' instead of 'cache' causes build errors.

Forgetting to specify the cache ID.

The mount type must be cache to enable caching. Other types do not support cache ID.

4fill in blank

hard

Fill both blanks to create a cache mount with an ID and target directory.

Docker

RUN --mount=type=[1],[2]=mycache,target=/root/.cache pip install -r requirements.txt

Drag options to blanks, or click blank then click option'

Acache

Bid

Cname

Dsource

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' or 'source' instead of 'id' for the cache identifier.

Using a mount type other than 'cache'.

The mount type must be cache and the cache identifier is set with id.

5fill in blank

hard

Fill all three blanks to create a cache mount with an ID, target directory, and sharing mode.

Docker

RUN --mount=type=[1],[2]=mycache,target=[3] pip install -r requirements.txt

Drag options to blanks, or click blank then click option'

Acache

Bid

C/root/.cache

Dshared

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'shared' as the target directory.

Mixing up the order of options.

Omitting the cache ID.

The mount type is cache, the cache ID is set with id, and the target directory is /root/.cache.

Practice

1. What is the main purpose of using --mount=type=cache in a Dockerfile during build?

easy

A. To save files between build steps and speed up future builds

B. To permanently store files in the final image

C. To share files between running containers

D. To mount host directories into the container at runtime

Solution

1. Step 1: Understand cache mount purpose

   The --mount=type=cache option is used during build to save files temporarily between build steps.

2. Step 2: Identify effect on build speed

   By saving files like package downloads, it avoids repeating work, making future builds faster.

3. Final Answer:

   To save files between build steps and speed up future builds -> Option A

4. Quick Check:

   Cache mount speeds builds = D [OK]

Hint: Cache mounts save build files to speed up next builds [OK]

Common Mistakes:

• Confusing cache mount with runtime volume mount
• Thinking cache mount stores files in final image
• Assuming cache mount shares files between containers

2. Which of the following is the correct syntax to use a cache mount in a Dockerfile RUN command?

easy

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

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

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

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

Solution

1. Step 1: Check correct mount syntax

   The correct syntax uses --mount=type=cache,target=... inside the RUN command.

2. Step 2: Validate options

   RUN --mount=type=cache,target=/root/.cache npm install matches the correct syntax exactly; others have wrong keywords or types.

3. Final Answer:

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

4. Quick Check:

   Correct cache mount syntax = A [OK]

Hint: Use --mount=type=cache,target=... exactly in RUN [OK]

Common Mistakes:

• Using --cache instead of --mount
• Omitting type=cache
• Using type=volume instead of cache

3. Given this Dockerfile snippet:

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

What is the expected effect on build time when running the second RUN command?

medium

A. The second RUN command will be slower because cache is cleared

B. The second RUN command will be faster because it reuses cached files

C. The second RUN command will fail due to cache mount conflict

D. The second RUN command ignores cache and runs normally

Solution

1. Step 1: Understand cache mount effect on repeated commands

   The cache mount stores files from the first pip install in /root/.cache.

2. Step 2: Predict second RUN command behavior

   The second RUN reuses cached files, so it runs faster by skipping downloads.

3. Final Answer:

   The second RUN command will be faster because it reuses cached files -> Option B

4. Quick Check:

   Cache reuse speeds second build step = B [OK]

Hint: Cache mount speeds repeated installs by reusing files [OK]

Common Mistakes:

• Assuming cache clears between RUN commands
• Thinking cache causes build failure
• Believing cache is ignored during build

4. You wrote this Dockerfile line:

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

But the cache is not speeding up builds. What is the likely mistake?

medium

A. The cache mount target directory is incorrect or not used by npm

B. You forgot to add --cache flag instead of --mount

C. Cache mounts only work with Python, not npm

D. You must run docker build with --no-cache option

Solution

1. Step 1: Check cache mount target relevance

   Cache mount must target the directory where npm stores cache files, usually /root/.npm, not /root/.cache.

2. Step 2: Understand npm cache usage

   If the target directory is wrong, npm won't use the cache, so builds stay slow.

3. Final Answer:

   The cache mount target directory is incorrect or not used by npm -> Option A

4. Quick Check:

   Correct cache target directory needed = C [OK]

Hint: Match cache target to tool's actual cache folder [OK]

Common Mistakes:

• Using wrong directory for cache mount target
• Confusing --cache and --mount flags
• Thinking cache mounts only work for some languages

5. You want to speed up a Docker build that uses apt-get update and apt-get install. How can you use cache mounts to optimize this build?

hard

A. Use RUN --mount=type=cache,target=/etc/apt before apt-get update

B. Use RUN --mount=type=cache,target=/var/lib/apt after apt-get install

C. Use RUN --mount=type=cache,target=/var/cache/apt before apt-get update

D. Use RUN --mount=type=cache,target=/usr/bin/apt before apt-get update

Solution

1. Step 1: Identify apt cache directory

   The apt package manager stores downloaded package files in /var/cache/apt.

2. Step 2: Apply cache mount before update

   Mounting cache at /var/cache/apt before apt-get update saves downloaded package lists and speeds up future runs.

3. Final Answer:

   Use RUN --mount=type=cache,target=/var/cache/apt before apt-get update -> Option C

4. Quick Check:

   Cache apt downloads at /var/cache/apt = A [OK]

Hint: Cache package manager download folders to speed builds [OK]

Common Mistakes:

• Mounting wrong directories like /etc/apt or /usr/bin/apt
• Mounting cache after install instead of before update
• Confusing apt cache with config directories