Update docker image ID

[alnorth]

Looks like the ID for the Digital Ocean docker image has changed. This is the value I got from their API.

{
    "id": 8345531,
    "name": "Docker 1.3.2 on 14.04",
    "slug": "docker",
    "distribution": "Ubuntu",
    "public": true,
    "regions": [
        1,
        2,
        3,
        4,
        5,
        6,
        7,
        8,
        9
    ],
    "region_slugs": [
        "nyc1",
        "ams1",
        "sfo1",
        "nyc2",
        "ams2",
        "sgp1",
        "lon1",
        "nyc3",
        "ams3"
    ]
},