Added docker examples. rtabmapviz: Fixed empty cloud warnings when not subscribing to any topics.

docker/melodic/Dockerfile

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+FROM ros:melodic-perception
+# install rtabmap packages
+RUN apt-get update && apt-get install -y \
+    ros-melodic-rtabmap \
+    ros-melodic-rtabmap-ros \
+    && rm -rf /var/lib/apt/lists/

docker/melodic/latest/Dockerfile

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+FROM ros:melodic-perception
+# install rtabmap packages
+RUN apt-get update && apt-get install -y \
+    ros-melodic-rtabmap-ros \
+    && apt-get remove -y \
+    ros-melodic-rtabmap \
+    && rm -rf /var/lib/apt/lists/
+
+RUN rm /bin/sh && ln -s /bin/bash /bin/sh
+
+WORKDIR /root/
+
+RUN git clone https://github.com/introlab/rtabmap.git
+
+# Build RTAB-Map project
+RUN source /ros_entrypoint.sh && \
+    cd rtabmap/build && \
+    cmake .. && \
+    make -j$(nproc) && \
+    make install && \
+    cd ../.. && \
+    rm -rf rtabmap && \
+    ldconfig
+
+RUN source /ros_entrypoint.sh && \
+    mkdir -p catkin_ws/src && \
+    cd catkin_ws/src && \
+    catkin_init_workspace && \
+    git clone https://github.com/introlab/rtabmap_ros.git && \
+    cd .. && \
+    catkin_make -DCMAKE_INSTALL_PREFIX=/opt/ros/melodic install && \
+    cd && \
+    rm -rf catkin_ws