Update Dockerfile to add ca-certificates
Update Dockerfile to add ca-certificates

If you add this package, you can keep HTTP between servers and apt-cacher-ng (and caching packages) and in same time, you give capabilities for apt-cacher-ng to download packages on HTTPS repository. 

for exemple:
* on all servers: 
deb http://download.docker.com/linux/debian stretch stable
* on apt-cacher-ng:
Remap-docker: download.docker.com ; https://download.docker.com
sameersbn/docker-apt-cacher-ng