Every line of a Dockerfile is actually an image/layer by itself.
Modify for instance the last bit of the previous image (let’s change the image URL) and rebuild it (even with a different name/tag):
FROM ubuntu:18.04 MAINTAINER Toni Hermoso Pulido <firstname.lastname@example.org> WORKDIR ~ RUN apt-get update && apt-get -y upgrade RUN apt-get install -y wget ENTRYPOINT ["/usr/bin/wget"] CMD ["https://cdn-images-1.medium.com/max/1600/1*_NQN6_YnxS29m8vFzWYlEg.png"]
docker build -t mytestimage2 .
It will start from the last line. This is OK most of the times and very convenient for testing and trying new steps, but it may lead to errors when versions are updated (either FROM image or included packages). For that it is benefitial to start from scratch with
docker build --no-cache -t mytestimage2 .