It was removed from the official mirror, because backports is not a part
of the LTS architecture. See [1] and [2] ('Deprecation of LTS support
for backports') for more information.

The reason to update the image is that doing `apt-get update` from the
image (say, from a prebuild script) now leads to the following errors:

 | Err http://ftp.debian.org jessie-backports/main Sources
 |   404  Not Found
 | Err http://ftp.debian.org jessie-backports/main amd64 Packages
 |   404  Not Found

[1]: https://lists.debian.org/debian-devel-announce/2019/03/msg00006.html
[2]: https://backports.debian.org/

It was removed from the official mirror, see [1] and links inside for
more information.

[1]: debuerreotype/docker-debian-artifacts#66

On 15.09.2019 released the new CentOS 7.7 in which repositories
devtoolset-6* packages were removed. Also repodata files were removed
at CentOS-SCLo-scl and CentOS-SCLo-scl-hp sources repositories.
To fix these issues the new devtoolset-8* packages were set instead
of devtoolset-6* and CentOS-SCLo-scl and CentOS-SCLo-scl-hp sources
repositories were removed from YUM configuration.

Added CentOS 8 image named as el-8 with the following differences from
CentOS 7:
(check https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html-single/considerations_in_adopting_rhel_8/index )
 - added PowerTools repository as suggested at
   https://fedoraproject.org/wiki/EPEL
 - removed curl which is allready installed at the parent image;
 - at CentOS 8 the localization environment changed to "C";
   (needed by backport repository);
 - removed pygpgme which was removed at CentOS 8 (check A.3.
   "Removed packages part");
 - saved backport repository due to python2 still in use while CentOS 8
   changed its packages to python3 and we need to backport python-gevent
   package;
 - cmake28 and cmake3 removed due to CentOS 8 installs 3.11 version;
 - removed devtoolset and scl* repositories due to devtoolset-8 is
   already in the standard repositories and scl* repositories still
   not available.

Created the new Dockerfile based on Fedora 30 image with added
backport repository https://packagecloud.io/packpack/backports
for packages python2-gevent and python2-greenlet which Fedora 31
does not provide.

Now it is more or less same as Fedora 31 Dockerfile.

This commit allows to successfully build the image. Before it fails,
because there is no more pygpgme package in Fedora Rawhide repositories.

Commented backports repository enabling, because the packagecloud.io
script detects the operating system as fedora/32 and reports that does
not support it.

The problem with Python 3 was resolved a long time ago, so the
workaround for it is not needed anymore.

Added docker files to build images for
the latest opensuse 'leap' versions:
15.0, 15.1, 15.2

It seems the problem appears when centos/7 mirrors starts to point to
centos/7.8.* directory (centos/7.8.2003 now).

If the problem will be resolved on CentOS infrastructure side, the
workaround should be removed.

Reported-by: Dmitriy Koltsov.

Created the new Dockerfile to build Fedora 32,
based on Fedora 31 image.