Fix docker-compose start error (celery#4691)

Running docker-compose on a clean checkout currently fails with a
validation error that `services.celery.build.args` contains null.
Referencing the `CELERY_USER` environment variable defined in `.env`
does not fix this since environment variables are only applied at
container run-time, not build-time. To avoid potential confusion due to
this, it's easiest to just hard-code the `CELERY_USER` variable in the
docker-compose file.

Author: c-w

docker/.env

@@ -23,7 +23,6 @@ RUN apt-get update && apt-get install -y \
 # unrelated elements, grouping them together makes the Dockerfile read better.
 ENV PROVISIONING /provisioning
 
-# This is provisioned from .env
 ARG CELERY_USER=developer
 
 # Check for mandatory build arguments

docker/Dockerfile

@@ -6,7 +6,7 @@ services:
       context: ..
       dockerfile: docker/Dockerfile
       args:
-        CELERY_USER:
+        CELERY_USER: developer
     environment:
       TEST_BROKER: pyamqp://rabbit:5672
       TEST_BACKEND: redis://redis
@@ -16,7 +16,7 @@ services:
       WORKER_LOGLEVEL: DEBUG
     tty: true
     volumes:
-      - ../celery:/home/$CELERY_USER/celery
+      - ../celery:/home/developer/celery
       # Because pytest fails when it encounters files from alternative python compilations,
       # __pycache__ and pyc files, PYTHONDONTWRITEBYTECODE must be
       # set on the host as well or py.test will throw configuration errors.