What are layers in a Dockerfile?

Layers in a Dockerfile are instructions that execute actions separately, creating intermediate images. Each layer represents changes in the image's filesystem.

When building a Dockerfile, each instruction (like RUN, COPY, ADD) creates a new layer on top of the previous one. Docker caches each layer, which speeds up subsequent builds when only part of the Dockerfile changes, as only layers after the changed instruction are rebuilt.

Advantages:

• Caching: Speeds up build times by reusing unchanged layers.
• Reuse: Common layers can be shared across different images.
• Image size: Removing unnecessary files in early layers reduces the final image size.

Example layers:

# Base image - first layer
FROM ubuntu:latest

# Install packages - second layer with filesystem changes
RUN apt-get update && apt-get install -y nginx

# Copy files - third layer with new files
COPY index.html /var/www/html/

Each instruction (FROM, RUN, COPY) creates a separate layer.