Use ARG with defaults for Kafka's versions and sha when building Kafka Docker image

Debezium's Dockerfile creates a nice clean Kafka image but has a small issue: the versions and the sha512 hash are created as environment variables which cannot be manipulated during docker build time (except maybe by regex replaces).

We can move these variables to ARG instructions with the same defaults and setting the {{ENV}}s from those build arguments. The behavior therefore stays exactly the same but it does allow users of this Dockerfile to inject different versions if need be.