Cache mount for faster builds in Docker - Step-by-Step Execution

Process Flow - Cache mount for faster builds

Start Docker Build

↓

Use Cache Mount in Dockerfile

↓

Check Cache Availability

↓

Reuse Cached

↓

Continue Build Steps

↓

Build Complete with Cache Speedup

The build starts, Docker checks if cache mount is available, reuses it if yes, else runs normally, speeding up builds by reusing cached data.

Execution Sample

Docker

FROM node:18
RUN --mount=type=cache,target=/root/.npm \
  npm install
COPY . .
RUN npm run build

This Dockerfile uses a cache mount for npm packages to speed up the install step during builds.

Process Table

Step	Action	Cache Mount Used?	Effect	Output
1	Start build	No	Initialize build	Build started
2	RUN npm install with cache mount	Yes	Reuses cached npm packages	npm packages installed quickly
3	COPY source files	No	Copies files to image	Files copied
4	RUN npm run build	No	Runs build script	Build completed
5	Finish build	N/A	Build finished with cache speedup	Image ready

💡 Build finishes after all steps, cache mount speeds up npm install step

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Cache Mount State	Empty	Populated with npm cache	Unchanged	Unchanged	Saved for next build
Build Progress	0%	33%	66%	100%	Complete

Key Moments - 3 Insights

Why does the npm install step run faster when using cache mount?

Because the cache mount stores npm packages from previous builds, so Docker reuses them instead of downloading again, as shown in step 2 of the execution table.

Does the cache mount affect the COPY or build script steps?

No, cache mount only speeds up the npm install step. COPY and build script run normally, as seen in steps 3 and 4.

What happens if the cache mount is empty or missing?

Docker runs npm install normally without cache, which takes longer. This is the 'No' branch in the concept flow.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the cache mount used?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Check the 'Cache Mount Used?' column in the execution table.

According to the variable tracker, what is the state of the cache mount after step 2?

A Empty

B Populated with npm cache

C Cleared

D Corrupted

💡 Hint

Look at the 'Cache Mount State' row after step 2 in the variable tracker.

If the cache mount was not used, how would the build time change?

A It would be faster

B It would be the same

C It would be slower

D Build would fail

💡 Hint

Refer to the concept flow where 'No' cache leads to running steps normally, which takes longer.

Concept Snapshot

Docker cache mount lets you save files between builds.
Use --mount=type=cache in RUN steps.
Speeds up repeated installs like npm.
Cache is stored outside image layers.
If cache missing, build runs slower.
Improves build efficiency and time.

Full Transcript

This visual execution shows how Docker cache mounts speed up builds. The build starts and reaches the npm install step. Docker checks if a cache mount is available. If yes, it reuses cached npm packages, making install faster. Then it copies source files and runs the build script normally. The cache mount state changes from empty to populated after step 2 and is saved for future builds. Key points: cache mount only affects the install step, not copy or build. If cache is missing, install runs slower. This method improves build speed by reusing data between builds.

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