Environment variables are in the .env file and loaded into the docker-compose.yml file at runtime.
The Dockerfile will need an ARG <value> line also

List all env variables and decide where they go
.env
env-ckan
ARG
ENV

NB:     .env variables can be referenced in the docker-compose.yml file (environment: part of the service)
        The docker-compose.yml file can pass these variables to the Dockerfile as ARG parameters
--------------------------- from OKFN ---------------------------
# DB image settings
POSTGRES_PASSWORD=ckan
DATASTORE_READONLY_PASSWORD=datastore

# Basic
CKAN_SITE_ID=default
CKAN_SITE_URL=http://ckan:5000
CKAN_PORT=5000
CKAN_SYSADMIN_NAME=ckan_admin
CKAN_SYSADMIN_PASSWORD=test1234
CKAN_SYSADMIN_EMAIL=your_email@example.com
TZ=UTC

# Database connections (TODO: avoid duplication)
CKAN_SQLALCHEMY_URL=postgresql://ckan:ckan@db/ckan
CKAN_DATASTORE_WRITE_URL=postgresql://ckan:ckan@db/datastore
CKAN_DATASTORE_READ_URL=postgresql://datastore_ro:datastore@db/datastore

# Test database connections
TEST_CKAN_SQLALCHEMY_URL=postgres://ckan:ckan@db/ckan_test
TEST_CKAN_DATASTORE_WRITE_URL=postgresql://ckan:ckan@db/datastore_test
TEST_CKAN_DATASTORE_READ_URL=postgresql://datastore_ro:datastore@db/datastore_test

# Other services connections
CKAN_SOLR_URL=http://solr:8983/solr/ckan
CKAN_REDIS_URL=redis://redis:6379/1
CKAN_DATAPUSHER_URL=http://datapusher:8800
CKAN__DATAPUSHER__CALLBACK_URL_BASE=http://ckan:5000

TEST_CKAN_SOLR_URL=http://solr:8983/solr/ckan
TEST_CKAN_REDIS_URL=redis://redis:6379/1

# Core settings
CKAN__STORAGE_PATH=/var/lib/ckan

CKAN_SMTP_SERVER=smtp.corporateict.domain:25
CKAN_SMTP_STARTTLS=True
CKAN_SMTP_USER=user
CKAN_SMTP_PASSWORD=pass
CKAN_SMTP_MAIL_FROM=ckan@localhost

# Extensions

CKAN__PLUGINS=envvars image_view text_view recline_view datastore datapusher
CKAN__HARVEST__MQ__TYPE=redis
CKAN__HARVEST__MQ__HOSTNAME=redis
CKAN__HARVEST__MQ__PORT=6379
CKAN__HARVEST__MQ__REDIS_DB=1


docker-compose.yml
DATASTORE_READONLY_PASSWORD=${DATASTORE_READONLY_PASSWORD}
POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
DATASTORE_READONLY_PASSWORD=${DATASTORE_READONLY_PASSWORD}
POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
PGDATA=/var/lib/postgresql/data/db