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Layers and cache

Learn about image layers and build cache.

Layers and the cache

Each instruction produces a layer and Docker caches the layers. If an instruction and its inputs (the copied files) didn't change, the layer comes from the cache (CACHED). When a layer changes, all layers after it are rebuilt:

 => CACHED [2/5] WORKDIR /app
 => CACHED [3/5] COPY requirements.txt .
 => CACHED [4/5] RUN pip install -r requirements.txt
 => [5/5] COPY . .                        # only the code changed

Turning off the cache

Sometimes the cache isn't wanted: when steps depending on an outside source like RUN apt-get update come from the cache, old packages can remain. --no-cache rebuilds all layers; --pull downloads a newer base image:

docker build --no-cache -t api:1.0 .
docker build --pull -t api:1.0 .
docker history api:1.0     # shows the layers and their sizes

Practice Clean build and layers

Write two commands: build api:1.1 without the cache and pulling the newest base image; then show this image's layer history.

Starter code

terminal
# buraya yaz

Sample solution

terminal
docker build --no-cache --pull -t api:1.1 .
docker history api:1.1

Expected output

[+] Building 6.3s (10/10) FINISHED
 => naming to docker.io/library/api:1.1 done
IMAGE          CREATED          CREATED BY                         SIZE
4f1e2d3c4b5a   3 seconds ago    CMD ["python" "app.py"]            0B
<missing>      3 seconds ago    RUN pip install --no-cache-dir …   41.2MB

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What happens if COPY . . is written before pip install?

  • The packages are reinstalled on every code change Correct answer
  • The build fails
  • The image shrinks
  • The cache works better

Explanation: Everything after the changed layer runs again.

2 Build without using the cache.

Question 
docker build ____ -t api:1.0 .
Answer 
docker build --no-cache -t api:1.0 .

Explanation: --no-cache rebuilds every layer.

3 Match.

Term Meaning
CACHED Came from the cache
--no-cache Rebuild all layers
--pull Update the base image
docker history Layer list

All lessons in Beginner

1 What is Docker?

  1. What is Docker?
  2. Containers and virtual machines
  3. Images and containers
  4. Docker architecture
  5. Use cases
  6. Summary - What is Docker

2 Installation and first commands

  1. Installing Docker
  2. docker version
  3. docker info
  4. The hello-world container
  5. Docker Desktop
  6. Summary - Installation

3 First container

  1. docker run basics
  2. Running in the background
  3. docker ps
  4. docker stop and rm
  5. docker logs
  6. Summary - First container

4 Dockerfile basics

  1. What is a Dockerfile?
  2. The FROM instruction
  3. The RUN instruction
  4. COPY and ADD
  5. CMD and ENTRYPOINT
  6. Summary - Dockerfile

5 Build and images

  1. docker build
  2. Tagging images
  3. Build context
  4. Layers and cache
  5. .dockerignore
  6. Summary - Build

6 docker run options

  1. Interactive mode
  2. Naming containers
  3. Restart policy
  4. Working directory
  5. Switching users
  6. Summary - docker run

7 Image management

  1. docker images
  2. docker rmi
  3. docker pull
  4. docker push
  5. docker tag
  6. Summary - Image management

8 Port mapping

  1. The port concept
  2. The -p flag
  3. Multiple ports
  4. Random ports
  5. IP binding
  6. Summary - Port mapping

9 Volumes and data

  1. Data persistence
  2. Named volumes
  3. The -v flag
  4. Bind mounts
  5. Volume management
  6. Summary - Volumes

10 Environment variables

  1. The -e flag
  2. The ENV instruction
  3. The ARG instruction
  4. env-file
  5. The WORKDIR instruction
  6. Summary - Environment

11 Project: docker-compose

  1. Compose design
  2. Service definition
  3. Database service
  4. Volume and network
  5. Putting it together - compose

12 Docker Hub and registry

  1. What is Docker Hub?
  2. docker login
  3. Searching images
  4. Official images
  5. Private registry
  6. Summary - Registry

13 Network basics

  1. Network drivers
  2. docker network ls
  3. Bridge network
  4. Container connectivity
  5. Publishing ports
  6. Summary - Network

14 Beginner final

  1. Challenge - Dockerfile
  2. Challenge - Build and run
  3. Challenge - Volume
  4. Challenge - Compose
  5. Challenge - Beginner project