validate mount path for tmpfs

There was no validation for `docker run --tmpfs foo`.

In this PR, only two obvious rules are implemented:
 - path must be absolute
 - path must not be "/"
We should add more rules carefully.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

integration-cli/docker_cli_create_unix_test.go

@@ -0,0 +1,43 @@
+// +build !windows
+
+package main
+
+import (
+	"strings"
+
+	"github.com/go-check/check"
+)
+
+// Test case for #30166 (target was not validated)
+func (s *DockerSuite) TestCreateTmpfsMountsTarget(c *check.C) {
+	testRequires(c, DaemonIsLinux)
+	type testCase struct {
+		target        string
+		expectedError string
+	}
+	cases := []testCase{
+		{
+			target:        ".",
+			expectedError: "mount path must be absolute",
+		},
+		{
+			target:        "foo",
+			expectedError: "mount path must be absolute",
+		},
+		{
+			target:        "/",
+			expectedError: "destination can't be '/'",
+		},
+		{
+			target:        "//",
+			expectedError: "destination can't be '/'",
+		},
+	}
+	for _, x := range cases {
+		out, _, _ := dockerCmdWithError("create", "--tmpfs", x.target, "busybox", "sh")
+		if x.expectedError != "" && !strings.Contains(out, x.expectedError) {
+			c.Fatalf("mounting tmpfs over %q should fail with %q, but got %q",
+				x.target, x.expectedError, out)
+		}
+	}
+}