Sorry, the confusion here is because my initial answer assumed we were talking about runner (agent) configuration parameters, not autoscaler configuration parameters.
The autoscaler has a parameter named DRONE_AGENT_ENV_FILE which is used to source environment variables from a file. The variables are used to configure the runner (agent). They are not exposed to pipeline steps. Pipeline steps do not inherit runner environment variables because they contain sensitive information (rpc secrets, global secrets, global registry credentials, etc)
The exec runner also has a documented parameter of the same name (this is the source of confusion). This parameter is used to load variables from file and pass to pipeline steps. https://docs.drone.io/runner/exec/configuration/reference/drone-runner-envfile/
The docker runner (agent) does not have this configuration parameter.
Hopefully this helps clarify.
given the complexity of the autoscaler configuration, I no longer believe environment variables are the best way to configure the autoscaler. All of the names and naming conflicts have gotten very confusing. In version 2.0 of the autoscaler I plan to switch to a configuration file (toml, yaml, hcl, etc) which should make configuration more expressive and easier to understand.
I still plan to support configuration from environment variables, but only for a subset of commonly used configuration parameters. Anything more advanced would need to use a configuration file.