Docker: added focal image, fixed bionic image not building with alicevision. Updated travis with focal build.

docker/focal/Dockerfile

@@ -0,0 +1,56 @@
+# Image: introlab3it/rtabmap:focal
+
+FROM ros:noetic-perception
+
+# Install build dependencies
+RUN apt-get update && \
+    apt-get install -y git software-properties-common ros-noetic-rtabmap-ros && \
+    apt-get remove -y ros-noetic-rtabmap && \
+    rm -rf /var/lib/apt/lists/
+
+WORKDIR /root/
+
+# GTSAM
+RUN add-apt-repository ppa:joseluisblancoc/gtsam-develop -y
+RUN apt install libgtsam-dev
+
+# libpointmatcher 
+RUN git clone https://github.com/ethz-asl/libnabo.git
+#commit Apr 25 2018
+RUN cd libnabo && \
+    git checkout 7e378f6765393462357b8b74d8dc8c5554542ae6 && \
+    mkdir build && \
+    cd build && \
+    cmake -DCMAKE_BUILD_TYPE=Release .. && \
+    make -j$(nproc) && \
+    make install && \
+    cd && \
+    rm -r libnabo
+RUN git clone https://github.com/ethz-asl/libpointmatcher.git
+#commit Jan 19 2018
+RUN cd libpointmatcher && \
+    git checkout 00004bd41e44a1cf8de24ad87e4914760717cbcc && \
+    mkdir build && \
+    cd build && \
+    cmake -DCMAKE_BUILD_TYPE=Release .. && \
+    make -j$(nproc) && \
+    make install && \
+    cd && \
+    rm -r libpointmatcher
+
+# Clone source code
+ARG CACHE_DATE=2016-01-01
+RUN git clone https://github.com/introlab/rtabmap.git
+
+RUN rm /bin/sh && ln -s /bin/bash /bin/sh
+
+# Build RTAB-Map project
+RUN source /ros_entrypoint.sh && \
+    cd rtabmap/build && \
+    cmake .. && \
+    make && \
+    make install && \
+    cd ../.. && \
+    rm -rf rtabmap && \
+    ldconfig
+