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.

Author: jen20

builtin/providers/docker/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
 }
 `