Modify Dockerfile to deal with archived buster

[jirijakes]

Seems like ruby:2.6.4 uses Debian Buster which was already archived and because of that the docker image cannot be built. This patch allows to use archived sources.

[jirijakes]

Error message encountered when trying to call apt-get update inside ruby:2.6.4:

root@8cc23e54d91b:/# apt-get update
Ign:1 http://security.debian.org/debian-security buster/updates InRelease
Ign:2 http://deb.debian.org/debian buster InRelease
Err:3 http://security.debian.org/debian-security buster/updates Release
  404  Not Found [IP: 146.75.46.132 80]
Ign:4 http://deb.debian.org/debian buster-updates InRelease
Err:5 http://deb.debian.org/debian buster Release
  404  Not Found [IP: 146.75.46.132 80]
Err:6 http://deb.debian.org/debian buster-updates Release
  404  Not Found [IP: 146.75.46.132 80]
Reading package lists... Done
E: The repository 'http://security.debian.org/debian-security buster/updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
[…]