Unrelated to the error, a few things that jumped out with your yaml. I noticed the yaml that you copy/pasted above is invalid. If you run it thought a yaml validator you will get an error:
ERROR:
while parsing a block mapping
in "<unicode string>", line 6, column 5:
image: plugins/docker
^
expected <block end>, but found '-'
in "<unicode string>", line 9, column 5:
- dockerfile: Dockerfile
^
I also noticed you seem to be mixing syntax from 0.8 and 0.9 in your yaml file. So once you resolve the yaml parsing issues, make sure you align your syntax with the version of Drone you are using.
and set up the secrets under the repositories
This makes me think you are using 0.8, because in 0.9 secrets are encrypted in the yaml