How upgrade a docker image without creating new image?

Question

It seems like the simplest question I can't find the answer for.

1. I build a docker image.
2. run docker images shows that it's present.
3. I then add a new line to the Docker file.
4. Then I rebuild the image.
5. The old image is still in docker images and there's now a new image.

How does one simply update the old image without creating a new one?

My docker command: docker build -t nick_app . --force-rm --no-cache

(Note: I just tossed those --force-rm command and --no-cache command' because it seemed like it would work.)

Before:

Dockerfile:

FROM ruby:2.3.3
RUN apt-get update -qq && apt-get install -y
CMD ["echo", "Sup"]

docker images command:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
nick_app            latest              421662154c3f        9 minutes ago       746.9 MB
ruby                2.3.3               d00ebde6a601        2 days ago               730.1 MB
hello-world         latest              c54a2cc56cbb        4 months ago          1.848 kB

After: Dockerfile:

FROM ruby:2.3.3
RUN apt-get update -qq && apt-get install -y \
   build-essential
CMD ["echo", "Sup"]

docker images command:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
nick_app            latest              bcf9b3667202        3 seconds  ago       746.9 MB
<none>              <none>              421662154c3f        10 minutes ago      746.9 MB
ruby                2.3.3               d00ebde6a601        2 days ago          730.1 MB
hello-world         latest              c54a2cc56cbb        4 months ago        1.848 kB

Every dockerfile change I make creates a new none image on rebuild. They're cumulative so every time I make a change and rebuild I get another none image.

How does one get rid of that intermediate none every time I make a dockerfile change?

Answer 1

Those <none>:<none> are actually the old version of your application, that has lost its named pointer, since you moved the nick_app:latest to your new build.

When building your image, I don't think that you can tell it to destroy the old image, it will simply create a new one. But there is a command that helps you to list or remove those dangling images :

docker images --filter dangling=true #lists all images that are dangling and has no pointer to it
docker rmi `docker images --filter dangling=true -q` #Removes all those images.