Check for no Cmd on exec create endpoint

Fixes #9414

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

integration-cli/docker_api_exec_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"testing"
+)
+
+// Regression test for #9414
+func TestExecApiCreateNoCmd(t *testing.T) {
+	defer deleteAllContainers()
+	name := "exec_test"
+	runCmd := exec.Command(dockerBinary, "run", "-d", "-t", "--name", name, "busybox", "/bin/sh")
+	if out, _, err := runCommandWithOutput(runCmd); err != nil {
+		t.Fatal(out, err)
+	}
+
+	body, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), map[string]interface{}{"Cmd": nil})
+	if err == nil || !bytes.Contains(body, []byte("No exec command specified")) {
+		t.Fatalf("Expected error when creating exec command with no Cmd specified: %q", err)
+	}
+
+	logDone("exec create API - returns error when missing Cmd")
+}