Artifact

Problem

You tested version 42 in TEST. For PROD, somebody rebuilds the source code. Are you actually deploying the exact thing you tested?

Solution — Immutable Build Artifact 

Build once:
source
  ↓
api:v42
  ↓
DEV → TEST → PROD
Promote the same artifact between environments.
Core idea: immutable artifacts separate building software from deploying it and ensure environments receive the same build.