I have a local Docker image that I need to push to a different repository, but the current name doesn't match the new registry path. I want to rename it without going through the entire build process again, as the source code hasn't changed. Is there a simple command in Software Development practices to just change the tag or the name of an existing image so I can push it to my private registry immediately?
3 answers
In Docker, you don't technically "rename" an image; instead, you create a new tag that points to the same Image ID. This is done using the docker tag command. The syntax is docker tag current_image_name:tag new_image_name:tag. Since both names point to the exact same layer hash, no new layers are created, and no rebuilding occurs. This is an essential time-saver in Software Development when you need to prepare an image for a specific environment, like moving an image from dev-build to prod-release, or preparing it for a specific registry like AWS ECR or Docker Hub.
Does creating a new tag automatically remove the old name, or will I end up with two identical images taking up double the disk space on my machine?
You can verify that they are the same by running docker images. You will see two different entries, but if you look at the "IMAGE ID" column, the alphanumeric strings will be identical.
I agree with Shirley. Checking the Image ID is the best way to gain confidence that you haven't accidentally triggered a heavy rebuild or duplicated your data during the Software Development process.
Patrick, that’s a very common concern! The good news is that because both tags point to the same underlying layers (the same Image ID), they don't take up any extra space. It's essentially just a pointer or an alias. If you want to get rid of the old name after tagging the new one, you can run docker rmi old_image_name. In Software Development environments with limited disk space, cleaning up these "alias" tags is a good habit, though the storage impact is practically zero as long as the Image ID remains the same.