Add configurable tag for use with auto_tag (#36)

Implements #30, allows setting a (single) tag to override the default of `latest` allows for the woodpecker-ci usecase of setting `next` as the tag for the default branch.

Co-authored-by: Gapodo <gapodo@datenclown.at>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/36
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: gapodo <gapodo@geekvoid.net>
Co-committed-by: gapodo <gapodo@geekvoid.net>

cmd/docker-buildx/config.go

@@ -127,6 +127,13 @@ func settingsFlags(settings *plugin.Settings) []cli.Flag {
 			Usage:       "generates tag names automatically based on git branch and git tag",
 			Destination: &settings.Build.TagsAuto,
 		},
+		&cli.StringFlag{
+			Name:        "tags.defaultName",
+			EnvVars:     []string{"PLUGIN_DEFAULT_TAG"},
+			Usage:       "allows setting an alternative to `latest` for the auto tag",
+			Destination: &settings.Build.TagsDefaultName,
+			Value:       "latest",
+		},
 		&cli.StringFlag{
 			Name:        "tags.suffix",
 			EnvVars:     []string{"PLUGIN_DEFAULT_SUFFIX", "PLUGIN_AUTO_TAG_SUFFIX"},