Bump node from 12.18.0-slim to 14.4.0-slim in /debian

Bumps node from 12.18.0-slim to 14.4.0-slim.

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

debian/Dockerfile

-FROM node:12.18.0-slim
+FROM node:14.4.0-slim
 
 # Build arguments to change source url, branch or tag
 ARG CODIMD_REPOSITORY=https://github.com/codimd/server.git