Setting build args in docker-compose.yml using env_file

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