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Two maintenance-related changes to the tutorial setup file.

1. Split up the reusable tutorial setup file into two based on whether the tutorial requires the latest/edge channel of Rockcraft.

2. Fix a bug in the setup instructions pertaining to Docker. Currently, the Docker setup code block looks like this:

sudo addgroup --system docker
sudo adduser $USER docker
newgrp docker
sudo snap disable docker
sudo snap enable docker

However, the newgrp command will open a new shell session. The disable and enable commands won't run if the user copies and pastes the entire command block. The fix splits up the command block into two different blocks so copying/pasting the whole block will work.

Additional reviewers: @jdkandersson