Overhaul how the storm2/ngx-voms-build image is produced

The initial aim of the changes was the upgrade of Openresty to version 1.19.9.1 (see issue #17).

But it has given the opportunity to review how the image was built. The changes are described in the commit messages.

The starting image is now storm2/build, but it could easily become directly CentOS, so that this image be not tied any more to the storm2 project.