Example launch files for dockerized watchdog & processes

[robbel]

This introduces the "dockerized" watchdog and monitored process example introduced in the Autoware workshop at IV 2020.

The choice of Docker for this example is because it facilitates use of cgroups (kernel control groups for resource allocation and isolation), and kernel namespaces for process isolation. The monitored entity can crash or be otherwise stopped/stalled and the Watchdog will respawn the entity.

To test the functionality in this PR:

• Install docker for your Linux distribution

• Check out this branch and edit the included Dockerfile as follows:

  diff --git a/Dockerfile b/Dockerfile
  index d6fa9fd..c3b79af 100644
  --- a/Dockerfile
  +++ b/Dockerfile
  @@ -6,7 +6,7 @@ LABEL stage=intermediate
  
  # Clone and build repository
  WORKDIR /root
  -RUN git clone https://github.com/ros-safety/software_watchdogs.git
  +RUN git clone -b experimental-relaunch https://github.com/ros-safety/software_watchdogs.git
  RUN . /opt/ros/foxy/setup.sh && \
   colcon build

  This will make sure that the correct branch is used in the next step (as long as this PR is not merged to master yet).

• Build the Docker image:

  docker build --no-cache . -t sw_watchdogs:latest

• Run the demo (watchdog_in_docker.launch.py) via:

  docker run -it --rm --name testing -v /var/run/docker.sock:/var/run/docker.sock \
  -v /usr/bin/docker:/usr/bin/docker sw_watchdogs:latest ros2 launch sw_watchdog watchdog_in_docker.launch.py

  This command brings up both the watchdog and the monitored application in two separate containers. See watchdog_in_docker.launch.py for details. Expect terminal output as follows after startup:

• Manually interfere with the monitored application, for example via:

  docker kill --signal=SIGINT talker

  This will kill the monitored entity (called talker) and force the watchdog to restart it shortly after. In the original terminal, this restart activity will be reflected as follows:

[robbel]

@togaen @gbiggs as discussed in the last ROS2 safety WG meeting, here is the dockerized version of the watchdog for testing cc @norro