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I have images based on microsoft/aspnet:4.6.2, when those (my) images were built, the microsoft/aspnet:4.6.2 was pulled down in order to build my own image. So, in my local computer I have the microsoft/aspnet:4.6.2 image. Let's say the base image gets updated, Microsoft finds a bug with the image and decided to make a fix maintaining the tag, so it's still called microsoft/aspnet:4.6.2 but it actually is a different image than it was when I built my own.

So I have two questions:
1. Everytime my image gets pulled down, it will get the base image as it was when I built my image, right? (it seems obvious but I need to check)
2. If I notice (web hook, trigger?) there's a newer version of microsoft/aspnet:4.6.2 can I just run the docker build command again and the newer image would get pulled down? Keep in mind the old base image is in my file system (called the same). Is Docker smart enough to realize that I have an older version of that base image and it'll download the latest version of it?

Hope I made myself clear

sebagomez
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  • Fixing a bug and releasing the new `image` with the same `tag` would be a bad thing to do. This doesn't guarantee reproducible behavior. You might want to take a look at [this](https://stackoverflow.com/a/26833005/1561148) and [this](https://medium.com/@mccode/the-misunderstood-docker-tag-latest-af3babfd6375) (about the `latest` tag). – tgogos Nov 22 '17 at 09:14

3 Answers3

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Your image, as downloaded by someone, will always remain the same. An image relies on specific layers to give the image it's SHA256 checksum. Modifying parent layers would modify the checksum used to reference the image, so that would become a new image. The only way for that image to change is if the image is referenced by a tag and the local tag changes, either manually or by pulling the image tag again.

docker build will use the a local image first by default. You either need to run docker build --pull, separately docker pull or docker rmi IMAGE for the build to use the latest tagged image.

The Docker Hub build service has a build feature to automatically rebuild when any specified image's are updated in the hub.

Matt
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use the —no-cache option during docker build if you need latest released base images else docker will always use the image available locally unless you do a cleanup post docker build

Jeel
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  1. yes. (internally it uses IMAGE ID, not the name, to refer base images)
  2. IIRC not smart by default. (there is a --pull switch in docker build )
Jokester
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