Build Slicer docker image with preinstalled elastix

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I have tried this approach and in the test it will fail with

CMake Error at CMakeLists.txt:17 (find_package):
  By not providing "FindSlicer.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "Slicer", but
  CMake did not find one.

  Could not find a package configuration file provided by "Slicer" with any
  of the following names:

    SlicerConfig.cmake
    slicer-config.cmake

  Add the installation prefix of "Slicer" to CMAKE_PREFIX_PATH or set
  "Slicer_DIR" to a directory containing one of the above files.  If "Slicer"
  provides a separate development package or SDK, be sure it has been
  installed.

The CMakeList.txt in my extension is:

cmake_minimum_required(VERSION 3.5)

project(Dosimetry4D)

#-----------------------------------------------------------------------------
# Extension meta-information
set(EXTENSION_HOMEPAGE "http://gitlab.com/opendose/opendose4d")
set(EXTENSION_CATEGORY "Dosimetry")
set(EXTENSION_CONTRIBUTORS "Alex Vergara Gil (INSERM) and Janick Rueegger (KSA)")
set(EXTENSION_DESCRIPTION "This extension contains the entire dosimetry workflow for a multipoint study")
set(EXTENSION_ICONURL "https://opendose.org/themes/default/images/opendose_favicon.png")
set(EXTENSION_SCREENSHOTURLS "http://www.example.com/Slicer/Extensions/Dosimetry/Screenshots/1.png")
set(EXTENSION_DEPENDS SlicerElastix) # Specified as a space separated string, a list or 'NA' if any

#-----------------------------------------------------------------------------
# Extension dependencies
find_package(Slicer REQUIRED)
include(${Slicer_USE_FILE})

#-----------------------------------------------------------------------------
# Extension modules
add_subdirectory(Dosimetry4D)
## NEXT_MODULE

#-----------------------------------------------------------------------------
include(${Slicer_EXTENSION_GENERATE_CONFIG})
include(${Slicer_EXTENSION_CPACK})

#-----------------------------------------------------------------------------
option(ENABLE_TESTS "Enable tests" OFF)
if (${ENABLE_TESTS})
    enable_testing()
endif()

but my extension is just a python module, Is there a way to avoid the find_package(Slicer REQUIRED) line??