feat issue #134: support linux mint by checking /etc/os-release for ID_LIKE and UBUNTU_CODENAME

[ptandler]

I just tried to install docker under mint 20 and this script failed: it was detected as debian.
Luckily, Mint provides information about the base distro in /etc/os-release:

NAME="Linux Mint"
VERSION="20 (Ulyana)"
ID=linuxmint
ID_LIKE=ubuntu  ### check this
PRETTY_NAME="Linux Mint 20"
VERSION_ID="20"
HOME_URL="https://www.linuxmint.com/"
SUPPORT_URL="https://forums.ubuntu.com/"
BUG_REPORT_URL="http://linuxmint-troubleshooting-guide.readthedocs.io/en/latest/"
PRIVACY_POLICY_URL="https://www.linuxmint.com/"
VERSION_CODENAME=ulyana
UBUNTU_CODENAME=focal  ### and this

I wasn't able to test this on all other distros. However, assuming that the entries in /etc/os-release are standardized, the changes should be safe.

[ptandler]

alright, seems like ubuntu and other use these fields as well :-( Thought it was easy.
Trying to fix.