Env Vars & Restart Policies

Configuration Without Rebuilding

The twelve-factor rule: same image everywhere, config injected via environment variables:

$ docker run -d --name api \
    -e DB_HOST=db.internal \
    -e LOG_LEVEL=debug \
    orders-api:v1

A missing env var is the single most common cause of a container that "starts then instantly dies" - the app can't find its config and exits. docker logs tells you which one.

Restart Policies

By default a container that dies stays dead, even after the Docker daemon or host reboots. Production containers need a policy:

Policy	Behavior
`no` (default)	never restart
`on-failure`	restart only if exit code ≠ 0
`always`	restart forever - even after you `docker stop` it (on daemon restart)
`unless-stopped`	restart after crashes/reboots, but respect a manual stop

$ docker run -d --restart unless-stopped --name worker myworker:v3

Tip: unless-stopped is the production default of choice: survives reboots, but when an engineer deliberately stops a container it stays stopped.

Goal: Practice both in the labs: Diagnose the Crash Loop (env vars + logs) and Survive the Reboot (restart policies).