Allow override for BASE_URL

Using this method the BASE_URL of an image can be overridden at
runtime allowing for users to specify their own CDN mirror.
I was actually just looking for this repo from which to provide a similar PR - glad to see you beat me to it.

Can we augment the getopt section above this to take a `-B` argument from which to pass the BASE_URL as well? I think it's best to avoid env-vars (as a primary means of input control) where possible and straightforward. This would also play well with your proposed change as the `BASE_URL`, being dynamically set based off of other variables, would need the `:-` treatment anyway.

```
        B) BASE_URL="${BASE_URL:-OPTARG}" ;;
```

The `USAGE` variable would also need to be updated.