Update gitpod.Dockerfile -- install olang.org/x/tools/cmd/deadcode

Author: devlights

.gitpod.yml

@@ -10,6 +10,7 @@ tasks:
       go install github.com/mgechev/revive@latest &&
       go install github.com/go-delve/delve/cmd/dlv@latest &&
       go install go.uber.org/nilaway/cmd/nilaway@latest &&
+      go install golang.org/x/tools/cmd/deadcode@latest &&
       task build
     command:
       go version