Implement containers pooling for faster commands startup

[vmagamedov]

Basically do this:

$ docker run --rm image:version sleep 3600
$ docker exec container-id cmd *args

Instead of:

$ docker run --rm image:version cmd *args

Benefits:

$ time docker exec 952fd2f07aaa true

real	0m0.082s
user	0m0.012s
sys	0m0.009s

Instead of:

$ time docker run --rm alpine:3.6 true

real	0m0.966s
user	0m0.013s
sys	0m0.008s

===> More than 10x faster