Merge pull request #51 from codimd/dependabot/docker/alpine/node-12.11.0-alpine

Bump node from 8.16.1-alpine to 12.11.0-alpine in /alpine

alpine/Dockerfile

-FROM node:8.16.1-alpine
+FROM node:12.11.0-alpine
 
 # Build arguments to change source url, branch or tag
 ARG CODIMD_REPOSITORY=https://github.com/codimd/server.git