ci: Pretty print the OS from an existing file
The CI already shows us the content of the $OS veriable (which is what we put in the /var/version file), for example:
$ docker pull "cangjians/build-essential:${OS}"
ubuntu-lts: Pulling from cangjians/build-essential
Status: Downloaded newer image for cangjians/build-essential:ubuntu-lts
So we don't need the /var/version file at all.
However, knowing this is ubuntu-devel
is great, but in a few months, will we remember what actual version was the Ubuntu LTS?
We can get a more interesting output from the /usr/lib/os-release file, which is a cross-distro standard:
$ docker run --rm "cangjians/libcangjie:${OS}" bash -c '. /usr/lib/os-release && echo $PRETTY_NAME'
Ubuntu 16.04.3 LTS