lib: allow relative paths in ImageManifest's App.Exec

[iaguis]

Since appc/spec#527 the spec allows relative
paths in App.Exec, so we don't need to prepend "/bin/sh -c".

[alban]

LGTM if it works.

[iaguis]

Hm, I pushed an old version, this is not needed. If both entrypoint and cmd are nil, append(entrypoint, cmd...) returns nil already.

[alban]

ok, LGTM as well.

[iaguis]

Tested with busybox (it uses a relative path):

$ docker2aci docker://busybox
Downloading sha256:eeee0535bf3: [==============================] 676 KB/676 KB 
Downloading sha256:a3ed95caeb0: [==============================] 32 B/32 B

Generated ACI(s):
library-busybox-latest.aci
$ actool cat-manifest --pretty-print library-busybox-latest.aci
{
    "acKind": "ImageManifest",
    "acVersion": "0.7.0",
    "name": "registry-1.docker.io/library/busybox",
    "labels": [
        {
            "name": "version",
            "value": "latest"
        },
        {
            "name": "os",
            "value": "linux"
        },
        {
            "name": "arch",
            "value": "amd64"
        }
    ],
    "app": {
        "exec": [
            "sh"
        ],
        "user": "0",
        "group": "0"
    },
    "annotations": [
        {
            "name": "created",
            "value": "2016-01-15T18:06:41Z"
        },
        {
            "name": "appc.io/docker/registryurl",
            "value": "registry-1.docker.io"
        },
        {
            "name": "appc.io/docker/repository",
            "value": "library/busybox"
        },
        {
            "name": "appc.io/docker/imageid",
            "value": "964092b7f3e54185d3f425880be0b022bfc9a706701390e0ceab527c84dea3e3"
        },
        {
            "name": "appc.io/docker/parentimageid",
            "value": "9e77fef7a1c9f989988c06620dabc4020c607885b959a2cbd7c2283c91da3e33"
        }
    ]
}
$ sudo rkt --insecure-options=image run --interactive library-busybox-latest.aci
rkt: using image from file /home/iaguis/work/coreos/go/src/github.com/coreos/rkt/build-rkt/bin/stage1-coreos.aci
rkt: using image from file library-busybox-latest.aci
/ # exit

[jonboulle]

can just inline this now..