Git SHA hash as environment varaible

I’d like to have my git sha hash of HEAD as an environment variable - any suggestions for how to accomplish this with convox?

Everything I’ve seen elsewhere involves setting a custom build arg, that is then read in during the build.

I’m thinking that a process that sets an environment variable (without promoting a release), then triggers the build would work (?), but was hoping for something more elegant - to do this I’d need a separate build step that runs the convox CLI and/or interacts with the API beyond the normal git push based workflow.