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.