container: general purpose multiarch docker

* new docker container using ubuntu focal (20.04)
* adds nuttx building for any supported target
* adds initial support for gazebo simulation via X11
* updates docker_run.sh script to initialize container
* NEEDS: updated ubuntu.sh script

How to run:
```
./Tools/docker_run.sh make all_variants_px4_fmu-v5x
```

Tools/setup/docker-entrypoint.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# Start virtual X server in the background
+# - DISPLAY default is :99, set in dockerfile
+# - Users can override with `-e DISPLAY=` in `docker run` command to avoid
+#   running Xvfb and attach their screen
+if [[ -x "$(command -v Xvfb)" && "$DISPLAY" == ":99" ]]; then
+	echo "Starting Xvfb"
+	Xvfb :99 -screen 0 1600x1200x24+32 &
+fi
+
+# Check if the ROS_DISTRO is passed and use it
+# to source the ROS environment
+if [ -n "${ROS_DISTRO}" ]; then
+	source "/opt/ros/$ROS_DISTRO/setup.bash"
+fi
+
+# Use the LOCAL_USER_ID if passed in at runtime
+if [ -n "${LOCAL_USER_ID}" ]; then
+	echo "Starting with UID : $LOCAL_USER_ID"
+	# modify existing user's id
+	usermod -u $LOCAL_USER_ID user
+
+	# run as user
+  exec gosu user "$@"
+else
+	exec "$@"
+fi