fix: run Docker container as non-root user (closes #34)

- Switch to gcr.io/distroless/cc-debian12:nonroot
- Add explicit USER 65534:65534 directive
- Add Docker security CI job verifying non-root UID, :nonroot base, and USER directive
- Document CIS Docker Benchmark compliance in SECURITY.md
- Add tests and edge cases for container security

src/lib.rs

@@ -13,6 +13,7 @@
 
 pub mod config;
 pub mod heartbeat;
+pub mod identity;
 pub mod memory;
 pub mod observability;
 pub mod providers;