SemVer calls it a pre-release identifier but you can in fact put anything you want in there that matches the syntactic rules, so you could just use your build id as a pre-release identifier. Or as the build id field, if you're not already using that.
The issue with doing it that way is package managers like NuGet can "hide" pre-releases--they treat them as different fundamental types of packages. In my case, I don't want that designation. I realize I could use pre-release identifiers but I'd rather have the artifact versioned "first-class," so to speak.
I would suggest putting it in the "build metadata" section then. If you're already putting a commit hash there, you can use the hyphen to separate it, like. 1.5.4+build123-b5bb9d80
The standard separator for build metadata fields is a period, since the section is ignored for version comparison it doesn’t really matter, but the spec does specify it (and some tools may display separate fields differently).
Oh you're right. I glanced at the spec and saw that it defined build metadata as [0-9A-Za-z-] but didn't realize that it was actually defining this as a dot-separated series of components where each component was [0-9A-Za-z-].
