What is the naming conventions of Docker tag?

[matfax (Matthias Fax)]

I’ts kind of disappointing that no colons or other special characters are allowed in the tag since the current best practice convention syntax that is even used by Docker images doesn’t give a clear distinction of separate dependency versions and version semantics. It’s a good practice to add dependencies to your tags (e.g., alpine3.8), in case that another image relies on it, but then again, most images also use the hyphen as separator so that you may end up with something like the Docker image “18.09-rc-git”. In this tag, it is unclear if “rc” and “git” are part of the version or represent a separate dependency. If colons were allowed, you would have a clearer tagging. For instance, in the format imagename:imageversion[:depversion(n++)], docker:18.09-rc:gitx.y.z:alpine3.8. That would benefit both the common dependency referencing practice and thus the security of the images, just as the readability of the tags.