install_on_linux.sh does not work with default compose install

[chripede]

The documentation says for the compose plugin to be installed into the users .docker folder. However the install_on_linux.sh script has to be run as root to allow for file creation in /usr/local/bin

The root user however doesn't have compose v2 installed, and the install script then fails (without any output).

[ndeloof]

cc @ulyssessouza