My answer should be valid for Docker image version(s): splunk/splunk:7.3.3(-redhat)
I've noticed this in entrypoint.sh and and the message during container startup which led me into "SPLUNK_PASSWORD" env. var.
then
echo "WARNING: No password ENV var. Stack may fail to provision if splunk.password is not set in ENV or a default.yml"
fi
This set of environment variables worked for me:
SPLUNK_START_ARGS: --accept-license --answer-yes
SPLUNK_ENABLE_LISTEN: 9997
SPLUNK_ADD: tcp 1514
SPLUNK_USER: root
#if custom admin password, env var required for >= 7.3.x
SPLUNK_PASSWORD: your-custom-password
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