Prerelease melodic Qt5 cmake error

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Hey, I am currently trying to do my prerelease tests for a melodic version of my meta-package robot_statemachine which was already released for kinetic.

Unfortunately, when running the prerelease tests for melodic, I encountered various errors related to Qt5 dependencies. The meta-package includes two packages which are basically plugins, one for rqt and the other for rviz. Unfortunately, when running the prerelease script on docker, both fail with the below errors:

rsm_rviz_plugins error:

CMake Error at CMakeLists.txt:31 (find_package):
  By not providing "FindQt5.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "Qt5", but
  CMake did not find one.

  Could not find a package configuration file provided by "Qt5" (requested
  version 5.9.5) with any of the following names:

    Qt5Config.cmake
    qt5-config.cmake

  Add the installation prefix of "Qt5" to CMAKE_PREFIX_PATH or set "Qt5_DIR"
  to a directory containing one of the above files.  If "Qt5" provides a
  separate development package or SDK, be sure it has been installed.

rsm_rqt_plugins error:

CMake Error at CMakeLists.txt:17 (find_package):
  By not providing "FindQt5Widgets.cmake" in CMAKE_MODULE_PATH this project
  has asked CMake to find a package configuration file provided by
  "Qt5Widgets", but CMake did not find one.

  Could not find a package configuration file provided by "Qt5Widgets" with
  any of the following names:

    Qt5WidgetsConfig.cmake
    qt5widgets-config.cmake

  Add the installation prefix of "Qt5Widgets" to CMAKE_PREFIX_PATH or set
  "Qt5Widgets_DIR" to a directory containing one of the above files.  If
  "Qt5Widgets" provides a separate development package or SDK, be sure it has
  been installed.

After some research I found this question. Following the answer I added a build dependency for qtbase5-dev. Unfortunately, this only lead to the following new error:

CMake Error at /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:83 (find_package):
  Could not find a package configuration file provided by "qtbase5-dev" with
  any of the following names:

    qtbase5-devConfig.cmake
    qtbase5-dev-config.cmake

  Add the installation prefix of "qtbase5-dev" to CMAKE_PREFIX_PATH or set
  "qtbase5-dev_DIR" to a directory containing one of the above files.  If
  "qtbase5-dev" provides a separate development package or SDK, be sure it
  has been installed.
Call Stack (most recent call first):
  CMakeLists.txt:6 (find_package)

Therefore, my question: Could someone point me out how to correctly include the Qt5 dependencies for rviz and rqt plugins in ROS melodic? Below you can see my CMakeLists and package files for the two packages throwing errors.

rsm_rqt_plugins: CMakeLists.txt:

cmake_minimum_required(VERSION 2.8.3)
project(rsm_rqt_plugins)

add_compile_options(-std=c++11)

find_package(catkin REQUIRED COMPONENTS
  roscpp
  rqt_gui
  rqt_gui_cpp
  rsm_msgs
  std_msgs
  std_srvs
)

set(CMAKE_AUTORCC ON)
find_package(cmake_modules REQUIRED)
find_package(Qt5Widgets REQUIRED)
include_directories(${Qt5Widgets_INCLUDE_DIRS} include)

###################################
## catkin specific configuration ##
###################################

catkin_package(
  INCLUDE_DIRS ${INC_DIR}
  LIBRARIES ${PROJECT_NAME}
  ${CMAKE_CURRENT_BINARY_DIR}/..
  ${CMAKE_CURRENT_BINARY_DIR}
  CATKIN_DEPENDS roscpp rqt_gui rqt_gui_cpp
)

###########
## Build ##
###########

## set variables for Statemachine Control node
set(SRCS_CONTROLS
  src/RSMControls.cpp
)
set(HDRS_CONTROLS
  src/RSMControls.h
)
set(UIS_CONTROLS
  src/rsm_controls.ui
)

set(INC_DIR
  include
  ${CMAKE_CURRENT_BINARY_DIR}
)

##########################################################################################
# qt5_wrap_cpp produces moc files for all headers listed
# qt5_wrap_ui produces .h files for all .ui files listed

## Statemachine Control:
qt5_wrap_cpp(MOCS_SRCS_CONTROLS ${HDRS_CONTROLS})
qt5_wrap_ui(UI_HEADER_CONTROLS ...