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/Dockerfile

@@ -0,0 +1,55 @@
+#
+# PX4 base development environment
+#
+
+FROM ubuntu:20.04
+LABEL maintainer="Daniel Agar <daniel@agar.ca>, Ramon Roche <mrpollo@gmail.com>"
+
+ENV DEBIAN_FRONTEND noninteractive
+ENV LANG C.UTF-8
+ENV LC_ALL C.UTF-8
+
+# Installing required utilities
+RUN apt-get update && apt-get -y --quiet --no-install-recommends install \
+  ca-certificates \
+  gnupg \
+  lsb-core \
+  lsb-release \
+  sudo \
+  software-properties-common \
+  wget \
+  gosu \
+  ;
+
+# Install PX4 Requirements
+COPY Tools/setup/requirements.txt /tmp/requirements.txt
+COPY Tools/setup/ubuntu.sh /tmp/ubuntu.sh
+# We support pre-downloading the gcc arm none eabi compiler
+# to speed up build times, if the file is not present when
+# building, the ubuntu.sh script will download it from source
+COPY *gcc-arm-none-eabi-9-2020-q2-update-linux.tar.bz2 /tmp/gcc-arm-none-eabi-9-2020-q2-update-linux.tar.bz2
+# The PATH env is set within ubuntu.sh, but given how we
+# are running the image using `gosu` to avoid read-only problems
+# with the filesystem the env variable does not persist
+ENV PATH="/opt/gcc-arm-none-eabi-9-2020-q2-update/bin:$PATH"
+ENV PATH="/opt/jdk-14.0.2+12/bin:$PATH"
+RUN bash /tmp/ubuntu.sh --from-docker --with-java --with-rtps
+
+ENV DISPLAY :99
+
+ENV FASTRTPSGEN_DIR="/usr/local/bin/"
+ENV TERM=xterm
+ENV TZ=UTC
+
+# SITL UDP PORTS
+EXPOSE 14556/udp
+EXPOSE 14557/udp
+
+# create user with id 1001 (jenkins docker workflow default)
+RUN useradd --shell /bin/bash -u 1001 -c "" -m user && usermod -a -G dialout user
+
+# create and start as LOCAL_USER_ID
+COPY Tools/setup/docker-entrypoint.sh /usr/local/bin/entrypoint.sh
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
+
+CMD ["/bin/bash"]