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/docker.go

@@ -126,6 +126,10 @@ func commandBuild(build Build, dryrun bool) *exec.Cmd {
 		args = append(args, "-t", fmt.Sprintf("%s:%s", build.Repo, arg))
 	}
 
+	for _, l := range build.Labels.Value() {
+		args = append(args, "--label", l)
+	}
+
 	return exec.Command(dockerExe, args...)
 }