docker stack: setting environment variable from secrets

To elaborate on the original accepted answer, just change your docker-compose.yml file so that it contains this as your entrypoint:

version: "3.7"
  services:
    server:
      image: alpine:latest
      secrets:
        - test
      entrypoint: [ '/bin/sh', '-c', 'export TEST=$$(cat /var/run/secrets/test) ; source /entrypoint.sh' ]
    
secrets:
  test:
    external: true

That way you don’t need any additional files!