speedup docker ps

Fixes #2767 

    $ time docker ps  -a | wc -l
    434
    docker ps -a  0.00s user 0.06s system 67% cpu 0.094 total
    wc -l  0.00s user 0.00s system 4% cpu 0.093 total

@jpetazzo, it's for you ;)


ping @crosbymichael @creack