From this picture that you previously posted:
You would also want to follow the format of the first original line, as mentioned here:
So yours would probably need to look like this:
hooks:
after_code:
- exec:
cd: $home/plugins
cmd:
- git clone https://github.com/discourse/docker_manager.git
- git clone https://github.com/discourse/discourse-akismet.git
- git clone https://github.com/discourse/discourse-whos-online.git