services:
  postgres:
    image: my_node_app
    ports: -8080:8080
    environment:
      APP_PASSWORD: mypassword
      ...

#!/bin/bash
get_credential <server> <dev-environment> <role> <key>

FROM node:4-slim
MAINTAINER myself

# ... do Dockerfile stuff

# TRIAL #1: run a bash script to set the environment varable --- UNSUCCESSFUL!
COPY set_en_var.sh /
RUN chmod +x /set_en_var.sh
RUN /bin/bash /set_en_var.sh

# original entry point
#ENTRYPOINT ["node", "mynodeapp.js", "configuration.js"]

# TRIAL #2: use a bash script as entrypoint that sets
# the environment variable and runs my node app .            --- UNSUCCESSFUL TOO!
ENTRYPOINT ["/entrypoint.sh"]

. mybashclient.sh
cred_str=$(get_credential <server> <dev-environment> <role> <key>
export APP_PASSWORD=( $cred_str )

# run the original entrypoint command
node mynodeapp.js configuration.js

. mybashclient.sh
cred_str=$(get_credential <server> <dev-environment> <role> <key>
export APP_PASSWORD=( $cred_str )