Ensure docker group is created when docker snap is installed

By default, Docker is only accessible with root privileges (sudo). We want to be able to use Docker commands as a regular user to let devpack push images to local registry:
```
$ sudo addgroup --system docker
$ sudo adduser $USER docker
$ newgrp docker
```

Locate the code path that installs the Docker snap and review how post-install setup is handled. Verify that the docker group is created and the current user can run Docker commands without sudo, including pushing an image to the local registry.

Written by the indexing model from the issue text.