Explicitly set uid / gid of clickhouse user & group to the fixed values 101.
It is especially important for rootless containers: in that case entrypoint can't do chown
and owners of mounted volumes should be configured externally.
We do that in advance at the begining of Dockerfile before any packages will be installed
to prevent picking those uid / gid by some unrelated software.
The same uid / gid (101) is used both for alpine and ubuntu.
Number 101 is used by default in openshift, and was used by all clickhouse-server docker images
before 20.10. In 20.11 it was changed (by accident) to 999.
