Links were pointing to the `debian` subdirectory. However, there docker build was refactored to use `redhat`: 1abf981a73, see https://github.com/scylladb/scylladb/pull/22910
No backport, just a README link fixes.
Closes scylladb/scylladb#28699
* github.com:scylladb/scylladb:
docs: fix path to the build_docker.sh which was moved from debian to redhat subdirectory
docs: fix link to docker build README.MD