Your .env is read. Why is the variable missing from the service?
A resolved image tag proves that Compose read .env. It does not prove that every value became a service environment variable.
Configuration, explained
Understand what Compose will run.
Resolve environment variables, file paths and port mappings with small examples and results you can inspect.
Configuration checks on Compose 5.3.0
services:
probe:
image: busybox:${CC_IMAGE_TAG}
environment:
CC_ONLY: ${CC_ONLY:?Set CC_ONLY}CC_ONLY: not-automatically-injectedA variable is passed when you assign it.
The starting library
Start with the symptom.
Each guide includes a minimal example, the observed result and the limits of the test.
env_file loads your app settings, but ${…} still uses a fallback
Service environment files and the inputs used to render compose.yaml are different layers. Test both in a single resolved model.
Changed the value, got the old one? Trace Compose’s two precedence steps
First resolve the expression. Then inspect the environment assigned to the service. A literal value and ${CC_LEVEL} behave differently.
Which .env is read when you use docker compose -f?
Three paths that look similar are resolved from different places. Make the input explicit and verify the resulting model.
16 model checks · 12 Sep 2026 · no containers startedInspect the evidence