doc: update docker.md & linux-command.md & nginx.md (#838)

* doc: update linux-command.md

* 增加代理构建

* 删除上次提交多余

docs/docker.md

@@ -163,6 +163,7 @@ $ docker build - < Dockerfile
 $ docker build - < context.tar.gz
 $ docker build -t eon/nginx-server .
 $ docker build -f myOtherDockerfile .
+$ docker build --build-arg https_proxy=127.0.0.1:8088 # 使用http代理构建
 $ curl example.com/remote/Dockerfile | docker build -f - .
 $ docker save -o <保存路径>/myimage.tar myimage:latest # 导出
 $ docker load -i <路径>/myimage.tar # 导入