provider/docker: Fixing the Docker Container Mount Test

```
make testacc TEST=./builtin/providers/docker TESTARGS='-run=TestAccDockerContainer_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/docker -v
-run=TestAccDockerContainer_ -timeout 120m
=== RUN   TestAccDockerContainer_basic
--- PASS: TestAccDockerContainer_basic (17.25s)
=== RUN   TestAccDockerContainer_volume
--- PASS: TestAccDockerContainer_volume (16.79s)
=== RUN   TestAccDockerContainer_customized
--- PASS: TestAccDockerContainer_customized (19.65s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/docker 53.712s
```

Author: stack72

builtin/providers/docker/resource_docker_container_test.go

@@ -29,8 +29,8 @@ func TestAccDockerContainer_volume(t *testing.T) {
 	var c dc.Container
 
 	testCheck := func(*terraform.State) error {
-		if len(c.Mounts) != 2 {
-			return fmt.Errorf("Incorrect number of mounts: expected 2, got %d", len(c.Mounts))
+		if len(c.Mounts) != 1 {
+			return fmt.Errorf("Incorrect number of mounts: expected 1, got %d", len(c.Mounts))
 		}
 
 		for _, v := range c.Mounts {