ROS2 colcon build fails on demo pkg docker [closed]

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Hello,

when I try to build the demos pkg (https://github.com/ros2/demos) I get following error:

Starting >>> action_tutorials_interfaces Starting >>> dummy_map_server Starting >>> dummy_sensors Starting >>> pendulum_msgs Finished <<< dummy_map_server [14.6s]
Starting >>> composition Finished <<< dummy_sensors [15.2s]
Starting >>> demo_nodes_cpp --- stderr: composition
CMake Error at CMakeLists.txt:15 (find_package): By not providing "Findexample_interfaces.cmake" in CMAKE_MODULE_PATH this project has asked CMake to find a package configuration file provided by "example_interfaces", but CMake did not find one.

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

example_interfacesConfig.cmake
example_interfaces-config.cmake

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

Failed <<< composition [ Exited with code 1 ] --- stderr: demo_nodes_cpp
CMake Error at CMakeLists.txt:15 (find_package): By not providing "Findexample_interfaces.cmake" in CMAKE_MODULE_PATH this project has asked CMake to find a package configuration file provided by "example_interfaces", but CMake did not find one.

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

example_interfacesConfig.cmake
example_interfaces-config.cmake

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

Aborted <<< demo_nodes_cpp Aborted <<< pendulum_msgs
Aborted <<< action_tutorials_interfaces

Summary: 2 packages finished [19.1s] 1 package failed: composition 3 packages aborted: action_tutorials_interfaces demo_nodes_cpp pendulum_msgs 2 packages had stderr output: composition demo_nodes_cpp

Runing on the latest ros:dashing-ros-core-bionic docker image. Steps to reproduce:

source /opt/ros/dashing/setup.bash 
mkdir -p ros2_ws/src
cd ros2_ws/src
git clone https://github.com/ros2/demos
cd ..
colcon build --symlink

Thank you for your help

Edit:

Was missing this git repo: https://github.com/ros2/example_inter...