inspect: format the output of docker inspect

Prior to this patch, one would get the output of docker inspect xxx
as below:

user@server:/mnt$ docker inspect ubuntu
[{
    "Architecture": "amd64",

    ...

    "VirtualSize": 199257566
}
]user@server:/mnt$

The last ']' was on the same line with the prompt, i wonder if it is
really what we want it be, it is a little weird, so i add a '\n' to it.

Signed-off-by: Hu Keping <hukeping@huawei.com>