Base cgit on the Buster variant.

The image fails to build with the latest stable release (Bullseye) and
since I'm using it anymore, I'm not going to invest time in figuring
this out (it's either going back a version and seeing if that works or
removing the image altogether).

cgit/Dockerfile

 # hadolint ignore=DL3006
-FROM registry.shore.co.il/httpd
+FROM registry.shore.co.il/httpd:buster
 USER root
 # hadolint ignore=DL3008,DL3015
 RUN apt-get update && \