build-prod playbook failing

[Deleted user]

Trying to run build-prod.yml results in this error (excised the recursive middle for brevity):

TASK [Iterate over group variables] ****************************************************************************************************************************
fatal: [build]: FAILED! => {"msg": "An unhandled exception occurred while templating '{{ docker_tag }}'. Error was a <class 'ansible.errors.AnsibleError'>, original message: An unhandled exception occurred while templating '{{ docker_tag }}'. Error was a <class 'ansible.errors.AnsibleError'>, original message: An unhandled exception occurred while templating '{{ docker_tag }}'. Error was a <class 'ansible.errors.AnsibleError'>, original message: An unhandled exception occurred while templating '{{ docker_tag }}'. ... original message: recursive loop detected in template string: {{ docker_tag }}"}

[Deleted user]

Appears to be caused by line 20 in hosts.ini:

build docker_tag="{{ docker_tag }}" registry="" build_action="local" run_action=false