Add support for nerdctl run --labels

[soharaki]

Signed-off-by: Sho Haraki endurance.test.20380119@gmail.com
/cc @AkihiroSuda

[soharaki]

$ cat labels 
user=123461
role=cache

$ sudo nerdctl run -l foo=bar -l fuga=piyo --label-file=labels nginx
/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
...

$ sudo ctr c info 23938b33c7f53da7d7fc68e02b18107f58870e77e8907d6c0cc1cee4baf31670  | grep Labels -A 5
    "Labels": {
        "foo": "bar",
        "fuga": "piyo",
        "role": "cache",
        "user": "123461"
    },

[AkihiroSuda]

Thanks