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>

plugin/impl_test.go

@@ -13,10 +13,11 @@ var defaultSettings = Settings{
 		StoragePath: "/var/lib/docker",
 	},
 	Build: Build{
-		Context:    ".",
-		Tags:       *cli.NewStringSlice("latest"),
-		LabelsAuto: true,
-		Pull:       true,
+		Context:         ".",
+		Tags:            *cli.NewStringSlice("latest"),
+		TagsDefaultName: "latest",
+		LabelsAuto:      true,
+		Pull:            true,
 	},
 	DefaultLogin: Login{
 		Registry: "https://index.docker.io/v1/",