Docker Build Layers

19 January 2020 · learn++

Docker build layers are an amazing concept and can greatly speedup your development workflow.

docker build -t mydockerimage:latest .

Docker Layers

A layer in Docker is a snapshot of the most atomic part of a Docker build. This means, that Docker will not need to rerun the part of the build that is part of a layer.

Layers are created by commands that have an internally consistent state.

For example a RUN command does not get called again unless it depends on another non-deterministic directive in the Dockerfile.

Non-deterministic directives are for example:

COPY https://mylibrarydownload.org/my-lib.tar.gz

You can easily make this deterministic by downloading the file first and then using

ADD my-lib.tar.gz

This way, Docker can create a layer from it and cache the result.

Another example where this is helpful is apt-get:

You could write:

RUN apt-get install curl

and then later discover, you also need less.

Instead of adding less to the line like so:

RUN apt-get install curl less # don't do this!

you can add another apt-get RUN command like so:

RUN apt-get install curl
RUN apt-get install less

This way, Docker can use the cached layer from

RUN apt-get install curl

and just add

RUN apt-get install less

Also, if you upload to the Docker Hub or even your own container registry, you will be saving space for other containers that install the same layer.