Bad idea. Copy it to your home and make the changes in this file. “desktop” files in ~/.local/share/applications/ supersede files in the system applications folder.
An extremely bad idea. You should use an override with systemctl edit docker.service or maybe use systemctl edit --full docker.service to get an complet override service file in /etc/systemd/system.
Editing files in /usr is never a good way to change things, since these files get always replace with an update. Depending on the files, you need to do different things.