Launcher script breaks at high numbers of sites (#414)

Launcher script breaks at high numbers of sites (#414)

On a multi-site app configuration files such as containers/web.yml
grow with the number of sites. We discovered that the launcher script breaks
at around 100 sites due to "Argument List too long" when
commands such 'echo' are being referenced from the within the exec
call.

This workaround echoes the "$input" content outside the exec (which we verified
works fine for 300 sites) and then cats the file content inside the
exec. Thus bypassing the argument limitation. Finally, we remove the temporary file generated.