Add sorting for prefixed semver

Strips non-numeric prefixes when checking semver. This also adds a
secondary layer to the sort whereby identical semvers are sorted
relative to eachother lexigraphically.

This will ensure a more stable sort for non-versioned tags (eg. `latest`
or `edge`) as well as multiple variants tagged with the same number.
(eg `1.0.0-alpine` compared to `1.0.0-ubuntu`).

Fixes #758

pkg/registry/tags_sort.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"unicode"
 
 	"golang.org/x/mod/semver"
 )
@@ -22,9 +23,9 @@ func SortTags(tags []string, sortTag SortTag) []string {
 		return tags
 	case SortTagSemver:
 		semverIsh := func(s string) string {
-			if semver.IsValid(s) {
+			s = strings.TrimLeftFunc(s, func(r rune) bool {
-				return s
+				return !unicode.IsNumber(r)
-			}
+			})
 			if vt := fmt.Sprintf("v%s", s); semver.IsValid(vt) {
 				return vt
 			}
@@ -36,7 +37,13 @@ func SortTags(tags []string, sortTag SortTag) []string {
 			} else if c < 0 {
 				return false
 			}
-			return strings.Count(tags[i], ".") > strings.Count(tags[j], ".")
+			if c := strings.Count(tags[i], ".") - strings.Count(tags[j], "."); c > 0 {
+				return true
+			} else if c < 0 {
+				return false
+			}
+
+			return strings.Compare(tags[i], tags[j]) < 0
 		})
 		return tags
 	default:

pkg/registry/tags_test.go

@@ -60,6 +60,8 @@ func TestTagsSort(t *testing.T) {
 		"4.8.0",
 		"4.8.1",
 		"4.9.0",
+		"ubuntu-5.0",
+		"alpine-5.0",
 		"edge",
 		"latest",
 	}
@@ -103,6 +105,8 @@ func TestTagsSort(t *testing.T) {
 				"4.8.0",
 				"4.8.1",
 				"4.9.0",
+				"ubuntu-5.0",
+				"alpine-5.0",
 				"edge",
 				"latest",
 			},
@@ -141,16 +145,20 @@ func TestTagsSort(t *testing.T) {
 				"4.8.0",
 				"4.8.1",
 				"4.9.0",
+				"alpine-5.0",
 				"edge",
 				"latest",
+				"ubuntu-5.0",
 			},
 		},
 		{
 			name:    "sort reverse",
 			sortTag: registry.SortTagReverse,
 			expected: []string{
+				"ubuntu-5.0",
 				"latest",
 				"edge",
+				"alpine-5.0",
 				"4.9.0",
 				"4.8.1",
 				"4.8.0",
@@ -187,6 +195,8 @@ func TestTagsSort(t *testing.T) {
 			name:    "sort semver",
 			sortTag: registry.SortTagSemver,
 			expected: []string{
+				"alpine-5.0",
+				"ubuntu-5.0",
 				"4.21.0",
 				"4.21",
 				"4.20.1",