docker stack: setting environment variable from secrets

I found this neat extension to Alejandro's approach: make your custom entrypoint load from ENV_FILE variables to ENV ones:

environment:
  MYSQL_PASSWORD_FILE: /run/secrets/my_password_secret
entrypoint: /entrypoint.sh

and then in your entrypoint.sh:

#!/usr/bin/env bash

set -e

file_env() {
   local var="$1"
   local fileVar="${var}_FILE"
   local def="${2:-}"

   if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
      echo >&2 "error: both $var and $fileVar are set (but are exclusive)"
      exit 1
   fi
   local val="$def"
   if [ "${!var:-}" ]; then
      val="${!var}"
   elif [ "${!fileVar:-}" ]; then
      val="$(< "${!fileVar}")"
   fi
   export "$var"="$val"
   unset "$fileVar"
}

file_env "MYSQL_PASSWORD"

Then, when the upstream image changes adds support for _FILE variables, you can drop the custom entrypoint without making changes to your compose file.

You need modify docker compose to read the secret env file from /run/secrets. If you want to set environment variables via bash, you can overwrite your docker-compose.yaml file as displayed below.

You can save the following code as entrypoint_overwrited.sh:

# get your envs files and export envars
export $(egrep  -v '^#'  /run/secrets/* | xargs) 
# if you need some specific file, where password is the secret name 
# export $(egrep  -v '^#'  /run/secrets/password| xargs) 
# call the dockerfile's entrypoint
source /docker-entrypoint.sh

In your docker-compose.yaml overwrite the dockerfile and entrypoint keys:

version: '3.1'
#...
build:
  context: ./test
  dockerfile: Dockerfile
entrypoint: source /data/entrypoint_overwrited.sh
tmpfs:
  - /run/secrets
volumes:
  - /path/your/data/where/is/the/script/:/data/
environment:  
  user_name: admin
  eureka_password: /run/secrets/password
secrets:
  - password

Using the snippets above, the environment variables user_name or eureka_password will be overwritten. If your secret env file defines the same env vars, the same will happen if you define in your service some env_file.

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!