imagefilter: make short output format as unstable

This commit changes the "short" output format so that it is
marked as "unstable". There are some open questions about this
format and how it relates to e.g.
osbuild/osbuild-composer#4336
which adds very similar API is a slightly different format.

So to prevent users from relying on the API before that is resolved
we need to at least make them aware that this may still change.

pkg/imagefilter/formatter.go

@@ -87,6 +87,12 @@ type textShortResultsFormatter struct{}
 func (*textShortResultsFormatter) Output(w io.Writer, all []Result) error {
 	var errs []error
 
+	// deliberately break the yaml until the feature is stable, there
+	// are open questions, e.g. how this relates to:
+	// https://github.com/osbuild/osbuild-composer/pull/4336
+	// which adds a similar but slightly different API
+	fmt.Fprint(w, "@WARNING - the output format is not stable yet and may change\n")
+
 	outputMap := make(map[string]map[string][]string)
 	for _, res := range all {
 		if _, ok := outputMap[res.Distro.Name()]; !ok {

pkg/imagefilter/formatter_test.go

@@ -73,23 +73,23 @@ func TestResultsFormatter(t *testing.T) {
 		{
 			"short",
 			[]string{"test-distro-1:qcow2:test_arch3"},
-			"test-distro-1:\n  qcow2: [ test_arch3 ]\n",
+			"@WARNING - the output format is not stable yet and may change\ntest-distro-1:\n  qcow2: [ test_arch3 ]\n",
 		},
 		{
 			"short",
 			[]string{
 				"test-distro-1:qcow2:test_arch3",
 				"test-distro-2:qcow2:test_arch3",
 			},
-			"test-distro-1:\n  qcow2: [ test_arch3 ]\ntest-distro-2:\n  qcow2: [ test_arch3 ]\n",
+			"@WARNING - the output format is not stable yet and may change\ntest-distro-1:\n  qcow2: [ test_arch3 ]\ntest-distro-2:\n  qcow2: [ test_arch3 ]\n",
 		},
 		{
 			"short",
 			[]string{
 				"test-distro-1:test_type:test_arch",
 				"test-distro-1:test_type:test_arch2",
 			},
-			"test-distro-1:\n  test_type: [ test_arch, test_arch2 ]\n",
+			"@WARNING - the output format is not stable yet and may change\ntest-distro-1:\n  test_type: [ test_arch, test_arch2 ]\n",
 		},
 	} {
 		res := make([]imagefilter.Result, len(tc.fakeResults))