Fix container ID extraction from /proc/self/mountinfo file

[buchdag]

This PR fixes #355

On some systems both /proc/1/cpuset and /proc/self/cgroup are left empty, leaving only /proc/self/mountinfo to retrieve the ID of the container docker-gen is running inside of. This file differs slightly from the two others and the GetCurrentContainerID() and matchContainerID() functions had to be rewritten to

1. process each file separately
2. use a different regexp for /proc/self/mountinfo

in order to avoir retrieving a 64 characters ID that is not the container ID.

[multicast]

Matching container ID on the /etc/hostname in the mountinfo file instead of the first (in my case, overlay fs) may help.

[buchdag]

That's more or less what the PR does, if matchContainerID() identifies the starting pattern of a /proc/<pid>/mountinfo files, it preprends containers/ to the regular expression, so that the function will match one of the lines with the correct ID instead of the first one.

It should work on setups similar to yours but that's unfortunately still a brittle solution without any guaranteed stability.