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Docker Layers

Updated Mar 11, 2022 ·

Overview​

Docker images are made up of layers. Each layer represents a change or command in the Dockerfile. Layers can be cached or reused to optimize builds.

  • Layers referencing common libraries (e.g., pip install requests)
  • These libraries can be reused across images.
  • Order and specifics of commands in the Dockerfile affect layer reuse.

Example:

RUN apt-get update  
RUN pip install requests

Why Care About Layers?​

Reusing layers improves efficiency.

  • Faster builds: Cached layers reduce the time needed to build an image.
  • Smaller storage use: Shared layers save disk space.

Inspecting Docker Image Layers​

To examine layers in an image, use the docker image inspect command.

  • Provides image details: configuration, architecture, OS, and layers.
  • Look for the RootFS Layers section to see the layers.

Example:

docker image inspect <image-id>

Example: Inspecting Postgres Layers​

The docker image inspect command shows layers in a Postgres image.

Command:

docker image inspect postgres:latest

Key details:

  • RootFS section lists all layers by their IDs.
  • Layers represent changes made to build the image.

Using jq for JSON Parsing​

The jq tool simplifies analyzing JSON outputs, like those from docker image inspect.

  • Parses and queries specific sections of JSON data.
  • Helps extract only the needed information.

Example usage:

docker image inspect <image-id> | jq '.'

jq Recipes with Docker​

Use jq to query specific image data.

  • To fetch the RootFS layers:

    docker image inspect <image-id> | jq '.[0].RootFS'

  • To count the number of layers:

    docker image inspect <image-id> | jq '.[0].RootFS.Layers | length'

Recipes like these make it easier to extract targeted data from complex JSON outputs.

Advanced jq Recipe Example​

Count layers in an image:

  1. Use jq to locate Layers under RootFS.
  2. Calculate the array length to determine the number of layers.

Command:

docker image inspect <image-id> | jq '.[0].RootFS.Layers | length'

Output includes the number of layers, demonstrating the flexibility of jq.