Can't set splunk.user with defaults.yml because SPLUNK_USER is set

Same issue for all of these envvars below. No amount of DEBUG=true or ANSIBLE_EXTRA_FLAGS=-vvvvvvvvvv tells you that the .YAML directive is being overridden by the envvar, either.

$ docker inspect splunk/universalforwarder:8.2.10 | jq '.[0].Config.Env'
[
  "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
  "container=oci",
  "SCLOUD_URL=https://github.com/splunk/splunk-cloud-sdk-go/releases/download/v1.11.1/scloud_v7.1.0_linux_amd64.tar.gz",
  "PYTHON_VERSION=3.7.10",
  "PYTHON_GPG_KEY_ID=0D96DF4D4110E5C43FBFB17F2D347EA6AA65421D",
  "SPLUNK_HOME=/opt/splunkforwarder",
  "SPLUNK_GROUP=splunk",
  "SPLUNK_USER=splunk",
  "SPLUNK_ROLE=splunk_universal_forwarder",
  "SPLUNK_DEFAULTS_URL=",
  "SPLUNK_ANSIBLE_HOME=/opt/ansible",
  "SPLUNK_OPT=/opt",
  "ANSIBLE_USER=ansible",
  "ANSIBLE_GROUP=ansible",
  "CONTAINER_ARTIFACT_DIR=/opt/container_artifact"
]

Reproduce the configuration using the docker-splunk image, defaults.yml, and the SPLUNK_USER environment variable, then trace how the Ansible settings are applied. Done means a value supplied through defaults.yml is not silently overridden by the image environment, with the behavior verified for the other listed environment variables.