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>

cmd/docker-buildx/config.go

@@ -133,6 +133,19 @@ func settingsFlags(settings *plugin.Settings) []cli.Flag {
 			Usage:       "generates tag names with the given suffix",
 			Destination: &settings.Build.TagsSuffix,
 		},
+		&cli.StringSliceFlag{
+			Name:        "labels",
+			EnvVars:     []string{"PLUGIN_LABEL", "PLUGIN_LABELS"},
+			Usage:       "sets labels to use for the image",
+			Destination: &settings.Build.Labels,
+		},
+		&cli.BoolFlag{
+			Name:        "labels.auto",
+			EnvVars:     []string{"PLUGIN_DEFAULT_LABELS", "PLUGIN_AUTO_LABEL"},
+			Usage:       "generates labels automatically based on git repository info",
+			Value:       true,
+			Destination: &settings.Build.LabelsAuto,
+		},
 		&cli.StringSliceFlag{
 			Name:        "args",
 			EnvVars:     []string{"PLUGIN_BUILD_ARGS"},