fix(Dockerfile): pull nginx from public repo

Author: Ryan-Git

Dockerfile

@@ -1,3 +1,3 @@
-FROM docker.mobike.io/database/nginx:1.15.1-alpine
+FROM nginx:1.15.8-alpine
 COPY dist/gravity-admin /usr/share/nginx/html
 COPY drc.conf /etc/nginx/conf.d/default.conf