running a bash client at docker creation to set an environment variable

From examples I've seen one can set environment variables in docker-compose.yml like so:

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

For security reasons, my use case requires me to fetch the password from a server that we have a bash client for:

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

In docker documentation, I found this, which says that I can pass in shell environment variable values to docker compose. So I can run the bash client to grab the passwords in my starting shell that creates the docker instances. However, that requires me to have my bash client outside docker and inside my maven project.

Another way to do this would be to run/cmd/entrypoint a bash script that can set environment variable for the docker instance. Since my docker image runs node.js, currently my Dockerfile is like this:

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"]

Here is the code for 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

And here is code for my set_en_var.sh:

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

So 2 questions:

1. Which is a better choice, having my bash client for password live inside docker or outside docker?
2. If I were to have it inside docker, how can I use cmd/run/entrypoint to achieve this?