Fix exception when PortSpecs is = null

If you create an image where PortSpecs is null, the executor will throw an exception after doing a docker inspect. This will set port to None if the image's PortSpecs is null. 

Example config that causes the issue:
```
"config": {
        "Hostname": "2f3c78ed5540",
        "Domainname": "",
        "User": "",
        "Memory": 0,
        "MemorySwap": 0,
        "CpuShares": 0,
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": null,
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": null,
        "Dns": null,
        "Image": "27eb999a2f541a0243ba3168ec3778533d4e1b73c8e34727d77956b6f2fa6d95",
        "Volumes": null,
        "VolumesFrom": "",
        "WorkingDir": "",
        "Entrypoint": null,
        "NetworkDisabled": false
    },
```