Extraído de link
EnvironmentFile similar to Environment directive but reads the environment variables from a text file. The text file should contain new-line-separated variable assignments.
Exemplo:
[Service]
EnvironmentFile=/etc/envfile
Conteúdo de /etc/envfile
:
NODE_ENV=production
PORT=3000
HTTP_FORWARDED_COUNT=1
MONGO_URL=mongodb://127.0.0.1:27017/wow-doge
ROOT_URL=https://localhost
METEOR_SETTINGS={"privateKey": "ohnoyouseemyprivatekey!"}