Document auto_tag behaviour more explicite (#37)

close #31

Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/37
Reviewed-by: Lauris BH <lafriks@noreply.codeberg.org>

docs.md

@@ -40,6 +40,12 @@ It will automatically generate buildkit configuration to use custom CA certifica
 | `tag`/`tags`              | @".tags"          | sets repository tags to use for the image
 | `platforms`               | *none*            | sets target platform for build
 
+## auto_tag
+
+If set to ture, it will use the `default_tag` ("latest") on tag event or default branch.
+If it's a tag event it will also asume sem versioning and add taggs acordingly (`x`, `x.x` and `x.x.x`).
+If it's not a tag event, and no default branch, automated tags are skipped.
+
 ## Examples
 
 ```yml

plugin/impl.go

@@ -123,7 +123,7 @@ func (p *Plugin) Validate() error {
 	}
 
 	if p.settings.Build.TagsAuto {
-		// return true if tag event or default branch
+		// we only generate tags on default branch or an tag event
 		if UseDefaultTag(
 			p.settings.Build.Ref,
 			p.settings.Build.Branch,

plugin/tags.go

@@ -73,14 +73,10 @@ func DefaultTags(ref, defaultTag string) ([]string, error) {
 }
 
 // UseDefaultTag for keep only default branch for latest tag
+// return true if tag event or default branch
 func UseDefaultTag(ref, defaultBranch string) bool {
-	if strings.HasPrefix(ref, "refs/tags/") {
-		return true
-	}
-	if stripHeadPrefix(ref) == defaultBranch {
-		return true
-	}
-	return false
+	return strings.HasPrefix(ref, "refs/tags/") ||
+		stripHeadPrefix(ref) == defaultBranch
 }
 
 func stripHeadPrefix(ref string) string {