Unit docker.service could not be found

Here's the proper way to use snap:

ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ docker.help
Docker snap: Docker Linux container runtime.

Due to the confinement issues on snappy, it requires some manual setup to make docker-snap works on your machine.
We'll take you through the steps needed to set up docker snap work for you on ubuntu core and ubuntu classic.

On Ubuntu classic, before installing the docker snap,
please run the following command to add the login user into docker group.
    sudo addgroup --system docker
    sudo adduser $USER docker
    newgrp docker

On Ubuntu Core 16, after installing the docker snap from store,
you need to connect the home interface as it's not auto-connected by default.
    sudo snap connect docker:home :home

Then have fun with docker in snappy.

ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ sudo docker run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://cloud.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/engine/userguide/

ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ sudo docker run -it ubuntu bash
root@7d52680ee4bb:/# 
root@7d52680ee4bb:/# exit
exit
ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ sudo snap services
Snap    Service  Startup  Current
docker  dockerd  enabled  active
ubuntu@ip-172-31-14-19:~$ 
ubuntu@ip-172-31-14-19:~$ sudo snap services docker
Snap    Service  Startup  Current
docker  dockerd  enabled  active
ubuntu@ip-172-31-14-19:~$ 

the above is from AWS, but works just as well from standard Ubuntu.

and to start the service:

thufir@dur:~$ 
thufir@dur:~$ sudo snap services
Snap    Service  Startup  Current
docker  dockerd  enabled  inactive
thufir@dur:~$ 
thufir@dur:~$ sudo snap start docker
Started.
thufir@dur:~$ 
thufir@dur:~$ sudo snap services
Snap    Service  Startup  Current
docker  dockerd  enabled  inactive
thufir@dur:~$