Setting build args in docker-compose.yml using env_file - docker

Question

I'm trying to use Docker and Docker Compose to create a containerized app. I have a PubNub account, which allows me to use different API keys for different environments (dev, test, prod). To help me build images for this, I am trying to use build args set with an env_file.
It's not working.
WARNING: The PUB_KEY variable is not set. Defaulting to a blank string.
WARNING: The SUB_KEY variable is not set. Defaulting to a blank string.
Questions:
What mistake am I making in setting the build args?
How do I fix it?
Is this a good way to set ENV variables for the containers scan and flask?
At the very bottom is an IntelliJ IDE screenshot, or the text code is just below.
Here is the docker-compose.yml content:
version: '3.6'
services:
scan:
env_file:
- sample.env
build:
context: .
dockerfile: Dockerfile
args:
pub_key: $PUB_KEY
sub_key: $SUB_KEY
target: scan
image: bt-beacon/scan:v1
flask:
env_file:
- sample.env
build:
context: .
dockerfile: Dockerfile
args:
pub_key: $PUB_KEY
sub_key: $SUB_KEY
target: flask
image: bt-beacon/flask:v1
ports:
- "5000:5000"
And the Dockerfile:
# --- BASE NODE ---
FROM python:3.6-jessie as base
ARG pub_key
ARG sub_key
RUN test -n "$pub_key"
RUN test -n "$sub_key"
# --- SCAN NODE ---
FROM base as scan
ENV PUB_KEY=$pub_key
ENV SUB_KEY=$sub_key
COPY app/requirements.scan.txt /
RUN apt-get update
RUN apt-get -y install bluetooth bluez bluez-hcidump python-bluez python-numpy python3-dev libbluetooth-dev libcap2-bin
RUN pip install -r /requirements.scan.txt
RUN setcap 'cap_net_raw,cap_net_admin+eip' $(readlink -f $(which python))
COPY app/src /app
WORKDIR /app
CMD ["./scan.py", "$pub_key", "$sub_key"]
# -- FLASK APP ---
FROM base as flask
ENV SUB_KEY=$sub_key
COPY app/requirements.flask.txt /
COPY app/src /app
RUN pip install -r /requirements.flask.txt
WORKDIR /app
EXPOSE 5000
CMD ["flask", "run"]
Finally, sample.env:
# PubNub app keys here
PUB_KEY=xyz1
SUB_KEY=xyz2

Answer 1

env_file can only set environment variables inside a service container. Variables from env_file cannot be injected into docker-compose.yml itself.
You have such options (described there in detail):
inject these variables into the shell, from which you run docker-compose up
create .env file containing these variables (syntax identical to your sample.env)
Personally I would separate image building process and container launching process (take away image building responsibility from docker-compose to external script, then building process can be configured easily).