Update Dockerfile

Dockerfile

@@ -35,6 +35,7 @@ RUN code-server --install-extension /home/cph-ng-0.5.5.vsix
 RUN code-server --install-extension /home/vscode-luogu.vsix
 RUN code-server --install-extension hediet.vscode-drawio
 RUN code-server --install-extension formulahendry.code-runner
+RUN code-server --install-extension llvm-vs-code-extensions.vscode-clangd
 
 RUN git config --global user.name "Save-it"
 RUN git config --global user.email "lin2011746853@outlook.com"