Description

Docker Compose interpolates variables only once, at the time it loads each environment file. If a variable is overridden later, the interpolated value does not get updated — leading to stale values.

Is this an expected behavior or an issue?

Steps To Reproduce

Given these two environment files:

• .envs/local/postgres:

DB_USER=my_user
POSTGRES_USER="${DB_USER}"

• .envs/staging/postgres:

DB_USER=my_user_overridden
POSTGRES_USER="${DB_USER}"

and the following docker compose files:

• docker-compose.local.yml:

services:
  postgres:
    image: pgvector/pgvector:0.7.4-pg14
    env_file:
      - .envs/local/postgres

• docker-compose.staging.yml:

services:
  postgres:
    env_file:
      - .envs/staging/postgres

Running:

docker compose -f docker-compose.local.yml -f docker-compose.staging.yml run postgres env | grep USER

yields:

DB_USER=my_user_overridden
POSTGRES_USER=my_user

I expected:

DB_USER=my_user_overridden
POSTGRES_USER=my_user_overridden

Compose Version

Docker Compose version v2.34.0

Docker Environment

Anything else?

No response