Revert "Bump node from 12.16.2-slim to 14.4.0-slim in /debian"

This reverts commit 1786bd34.

CodiMD 1.6.0 is incompatible with Node version 14

debian/Dockerfile

-FROM node:14.4.0-slim
+FROM node:12.16.2-slim
 
 # Build arguments to change source url, branch or tag
 ARG CODIMD_REPOSITORY=https://github.com/codimd/server.git