Update Dockerfile

[caiyaonan]

修改 Dokcerfile Nginx镜像版本，否则由于镜像源的问题会出不兼容的的报错

=> ERROR [serverstatus stage-1 2/5] RUN mkdir -p /ServerStatus/server/ && ln -sf /dev/null /var/log/nginx/access.log && ln -sf /dev/nul 0.1s
=> CANCELED [serverstatus builder 2/6] RUN apt-get update -y && apt-get -y install gcc g++ make libcurl4-openssl-dev 0.0s

[serverstatus stage-1 2/5] RUN mkdir -p /ServerStatus/server/ && ln -sf /dev/null /var/log/nginx/access.log && ln -sf /dev/null /var/log/nginx/error.log:
0.042 ln: failed to access '/var/log/nginx/access.log/null': Not a directory