provider/docker: Refer to a tag instead of latest

This should make tests more stable going forward. Also switch out the
image used from Ubuntu to Alpine Linux to reduce required download size
during test runs.

resource_docker_image_test.go

@@ -17,7 +17,7 @@ func TestAccDockerImage_basic(t *testing.T) {
 					resource.TestCheckResourceAttr(
 						"docker_image.foo",
 						"latest",
-						"d52aff8195301dba95e8e3d14f0c3738a874237afd54233d250a2fc4489bfa83"),
+						"8dd8107abd2e22bfd3b45b05733f3d2677d4078b09b5edce56ee3d8677d3c648"),
 				),
 			},
 		},
@@ -44,8 +44,8 @@ func TestAddDockerImage_private(t *testing.T) {
 
 const testAccDockerImageConfig = `
 resource "docker_image" "foo" {
-	name = "ubuntu:trusty-20150320"
-	keep_updated = true
+	name = "alpine:3.1"
+	keep_updated = false
 }
 `