Inserting Serial/License Number into Docker Compose via a Swarm Secret

Has anyone been able to inject a Docker Swarm secret into a Docker Compose file's environmental settings?  So in the documentation they have "serial" as an environmental variable we can pass in via either an env_file or just directly into the compose file (or just putting it into the docker run commnand).

The issue is I can't use a Swarm secret and I don't see a way to inject it via the setupColdFusion.cfm file they want us to run as I don't see that capability via the Admin API.

Has anyone been able to avoid putting in your serial/license key in plain text on a Docker Compose or Environment file?  I've tried injecting it after runtime but it doesn't seem to work.  I am a bit of a Unix novice so really all I've tried is setting the environmental variable named serial and restarting CF.  I'm curious if anyone has had any success.

Thanks!