Implement option to add image labels and generate automatic labels (#19)

Fixes #16

Results into labels:
![attels](/attachments/3aa6641e-fe50-4e28-be97-64c0afe2c35a)

Co-authored-by: Lauris BH <lauris@nix.lv>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/19
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Co-committed-by: Lauris BH <lafriks@noreply.codeberg.org>

plugin/impl.go

@@ -49,6 +49,8 @@ type Build struct {
 	TagsAuto   bool            // Docker build auto tag
 	TagsSuffix string          // Docker build tags with suffix
 	Tags       cli.StringSlice // Docker build tags
+	LabelsAuto bool            // Docker build auto labels
+	Labels     cli.StringSlice // Docker build labels
 	Platforms  cli.StringSlice // Docker build target platforms
 	Args       cli.StringSlice // Docker build args
 	ArgsEnv    cli.StringSlice // Docker build args from env
@@ -98,6 +100,9 @@ func (p *Plugin) Validate() error {
 			return nil
 		}
 	}
+	if p.settings.Build.LabelsAuto {
+		p.settings.Build.Labels = *cli.NewStringSlice(p.Labels()...)
+	}
 
 	return nil
 }