What is a layer in a Dockerfile?
A layer in a Dockerfile is the result of executing one instruction in the Dockerfile. Each command (FROM, RUN, COPY, ADD, and so on) creates a new layer in the image.
How to understand this at the junior level
- An image is built in layers, like a cake -> each step adds a layer.
- Layers are cached -> if a layer has not changed, Docker does not rebuild it and takes it from the cache. This speeds up the build.
- Layers are immutable -> if something needs to change, Docker just creates a new layer on top, without changing the lower ones.
Example
Dockerfile
FROM ubuntu:22.04 # layer 1
RUN apt-get update # layer 2
COPY . /app # layer 3
RUN make build # layer 4As a result, the image consists of 4 layers.
Summary in one phrase
A layer is the immutable result of one Dockerfile instruction, which Docker caches and reuses when building.
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