Error building a custom KIE Server extension image for an external database using Cekit

Issue

Following the documentation "Building a custom KIE Server extension image for an external database" for building a custom KIE Server extension image for Microsoft SQL Server with 'make mssql', it raises the error below:

2021-01-18 09:38:54,184 cekit        ERROR    CEKit dependency: 's2i' was not found, please provide the 's2i' executable.
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cekit/cli.py", line 335, in run
    command.execute()
  File "/usr/lib/python2.7/site-packages/cekit/builder.py", line 26, in execute
    self.prepare()
  File "/usr/lib/python2.7/site-packages/cekit/test/behave_tester.py", line 46, in prepare
    self.dependency_handler.handle(self.test_collector, self.params)
  File "/usr/lib/python2.7/site-packages/cekit/tools.py", line 392, in handle
    self._handle_dependencies(o.dependencies(params))
  File "/usr/lib/python2.7/site-packages/cekit/tools.py", line 302, in _handle_dependencies
    self._check_for_executable(dependency, executable)
  File "/usr/lib/python2.7/site-packages/cekit/tools.py", line 349, in _check_for_executable
    raise CekitError(msg)
CekitError: CEKit dependency: 's2i' was not found, please provide the 's2i' executable.
make: *** [mssql] Error 1