correct, for security reasons, Drone never exposes credentials from the host to pipeline containers (unless explicitly configured to do so). If you need to provide credentials to the plugin it looks like you need to set the username and password.
image: plugins/kaniko
settings:
username: <username>
password: <password>
however, presumably you would want to source the values from secrets
image: plugins/kaniko
settings:
username:
from_secret: <username secret>
password:
from_secret: <password secret>