add help message when no arguments are provided (#455)

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

test/cli/packages_cmd_test.go

@@ -16,6 +16,15 @@ func TestPackagesCmdFlags(t *testing.T) {
 		env        map[string]string
 		assertions []traitAssertion
 	}{
+		{
+			name: "no-args-shows-help",
+			args: []string{"packages"},
+			assertions: []traitAssertion{
+				assertInOutput("an image/directory argument is required"),              // specific error that should be shown
+				assertInOutput("Generate a packaged-based Software Bill Of Materials"), // excerpt from help description
+				assertFailingReturnCode,
+			},
+		},
 		{
 			name: "json-output-flag",
 			args: []string{"packages", "-o", "json", request},

test/cli/power_user_cmd_test.go

@@ -12,6 +12,15 @@ func TestPowerUserCmdFlags(t *testing.T) {
 		env        map[string]string
 		assertions []traitAssertion
 	}{
+		{
+			name: "no-args-shows-help",
+			args: []string{"power-user"},
+			assertions: []traitAssertion{
+				assertInOutput("an image/directory argument is required"), // specific error that should be shown
+				assertInOutput("Run bulk operations on container images"), // excerpt from help description
+				assertFailingReturnCode,
+			},
+		},
 		{
 			name: "json-output-flag-fails",
 			args: []string{"power-user", "-o", "json", "docker-archive:" + getFixtureImage(t, "image-pkg-coverage")},