Update docker image ID

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"
		]
	},