What are layers in Docker? Why should the number of layers be minimized?

Docker layers are separate intermediate states of the filesystem created during the execution of each instruction in a Dockerfile (for example, RUN, COPY, ADD).

Each layer stores changes relative to the previous layer, and the final image is assembled from a sequence of such layers.

Minimizing the number of layers is important for several reasons:

• The size of the final image is reduced because there is less overhead data
• Build and load speeds of the image are increased
• Caching and reuse of layers are simplified

Therefore, multiple commands are often combined into a single RUN instruction using && to create fewer layers.

Example:

RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

instead of three separate RUN commands.