Add OS to docker info

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

This PR is to add the Operating System (that is, Linux distro) to the docker info output.
This is towards asking for a single command (`docker info`) when debugging issues.
This needs to be something like `if !bytes.HasSuffix(line, []byte{'/'}) && len(line) > 0 {` because the last line is empty.

example:
```
'14:name=systemd:/', len: 17
'13:hugetlb:/', len: 12
'12:net_prio:/', len: 13
'11:perf_event:/', len: 15
'10:bfqio:/', len: 10
'9:blkio:/', len: 9
'8:net_cls:/', len: 11
'7:freezer:/', len: 11
'6:devices:/', len: 11
'5:memory:/', len: 10
'4:cpuacct:/', len: 11
'3:cpu:/', len: 7
'2:cpuset:/', len: 10
'', len: 0
```