firefly_iii docker compose is broken on Debian Buster based pureos 9

These three lines in the docker.compose on pureos

      - ServerName={% if firefly_iii.domain %}{{ firefly_iii.domain }}{% else %}{{ firefly_iii.subdomain + "." + domain }}
{% endif %}
    networks:

Evaluates to:

ServerName=firefly3    networks

once installed on the server.

This causes the following error to occur when attempting to use docker-compose.

docker-compose -p firefly_iii up
WARNING: The Y variable is not set. Defaulting to a blank string.
ERROR: The Compose file './docker-compose.yml' is invalid because:
services.firefly_iii_app.environment contains {"ServerName=firefly3    networks": null}, which is an invalid type, it should be a string