Add main function to install script to not execute half-retrieved scripts

[vsund]

curl xyz | bash executes commands as they arrive. If there's a network error Bash might execute a half-retrieved script (which obviously isn't good :)). Docker's install script gets around this with:

# wrapped up in a function so that we have some protection against only getting
# half the file during "curl | sh"
do_install

We probably should add something similar :)