How to: What is `${DISPLAY:-:0.0}`?
`$DISPLAY` is the shell variable that refers to your X11 display server.

`${DISPLAY}` is the same, but allows you to join variables like this:

- e.g. `${DISPLAY}_${DISPLAY}` would print `:0.0_:0.0`
- e.g. `$DISPLAY_$DISPLAY`     would print `:0.0`

...because `$DISPLAY_` is not `$DISPLAY`

`${variable:-fallback}` allows you to set a "fallback" variable to be substituted if `$variable` is not set.

You can also use `${variable:=fallback}` to set that variable (in your current terminal).

In Docker-OSX, we assume, `:0.0` is your default `$DISPLAY` variable.

You can see what yours is

```bash
echo $DISPLAY
```

That way, `${DISPLAY:-:0.0}` will use whatever variable your X11 server has set for you, else `:0.0`