How can I hide/show the Ubuntu dock with a keyboard shortcut?

Not exactly what it's intended for, but you can use super+q to show the Ubuntu dock when it's hidden.

The actual use of the key combination is somewhat different, the super+q combination shows the dock with numbers next to each icon in the dock. Then you release q and press the number to launch or switch to the associated application.

If this feature is disabled, run the following to enable it first:

gsettings set org.gnome.shell.extensions.dash-to-dock hot-keys true