osbuild/osbuild v18 on GitHub

CHANGES WITH 18:

All the RHEL runners now always use platform-python. This is the
python3.6 based interpreter that provides a stable platform for
system software based on python to be used. It is also always
available, in contrast to the python3 binary, that needs to be
installed separately.
The org.osbuild.selinux stage now support label overwrites, i.e.
manually specifying the label specific files and directories.
Improvements to the testing infrastructure, including new tests for
the org.osbuild.selinux stage and the org.osbuild.ostree.commit
assembler. Additionally, the tests do not rely on the nbd kernel
module, which seems to have stability issues.

Contributions from: Christian Kellner

— Berlin, 2020-06-23