command not found after switching users

Using debian, (running as an image for docker container), I run these following commands.

user>>service 
(shows hints)
user>>su - root
root>>su - docker
user>>service 
(command not found)

So when logging to docker user, I can run the service command.
After switching to root and then back to docker user, the command is not found.

What is the explanation for this and what can I do to solve this problem?
is it a bug or a feature?