Why you should use container image labels Most of these items are well known and used in every Dockerfile, but since label metadata is not something required to run your containers, it is often overlooked. LABEL: Key/value pairs that are not used by the runtime engine USER: Which user to run the process underĮNV: Variable (and its value) to be set in the processes environmentĪRG: Build argument passed into the container build container and used like an environment variable in the scope of the buildĬMD: Command and/or parameters to use to start the process (ENTRYPOINT is similar) If you list the “history” of an image, you will often see zero-byte sized layers because they don’t contain filesystem changes, but rather, metadata to be used at runtime that are commonly added by Dockerfile commands:
#Docker tag software
RPM, APT, NPM, and most other packaging tools have similar metadata stored in them that is used by the tools in the process of installing or running the contained software or for utility purposes by repositories or runtime monitoring systems.Ĭontainer images have metadata stored in their layers too. jar. (FYI “POM” stands for Maven’s Project Object Model) jar built by the popular Maven build tool, there will usually be (among other things) a maven directory that has content such as the effective Maven pom.xml and pom.properties used to build the. are recognized and interpreted by the Java 2 Platform to configure applications, extensions, class loaders, and services”. If we open up a.
#Docker tag zip
zip archives, they all have a top-level META-INF directory that contains several files and directories that, per the Java 2 Platform spec, “. If you’ve built any kind of software package before, you know that - for the most part - their models include the software, configuration, and sometimes functional data as well as metadata about the package itself.įor example, while Java. They point to images that may have common filesystem layers but diverged at some point.Book a live demo Docker / OCI image metadata explained In the same way trusty and xenial do not point to the same image.
#Docker tag download
This way and because I know that the latest (stable) version of Ubuntu is xenial and the version number is 16.04, I can download this specific image from the Docker Hub using either of these terms or tags. With Ubuntu: xenial, latest and 16.04 are tags that point to the same location, these are just many ways to refer to the same image. Multiple tags can point to one container image. For example these are all valid tags for Ubuntu (see docker hub's page): rolling Taking Ubuntu as an example, tags are used to refer to specific releases of the operating system and there can be plenty. You tag a release for your image and push changes to your remote repository. It is no different than tagging a release with git if you are familiar with it but you are tracking changes to your image in this case. When a user downloads your image from a registry like Docker Hub or the Google Container Registry, they can easily associate which version they are downloading from the tag. Because your image evolves over time and sees more layers being added to form a new image, tags are also a convenient way to do versioning.
In short, tags are used for convenience in order to identify an image (which is a combination of filesystem layers).
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