Separate Ubuntu .deb packages per Ubuntu release (#4357)

Our .deb packages are now very Ubuntu-specific and are packages based on our CI builds.

.docker/Dockerfile-ubuntu-20.04-build

@@ -0,0 +1,54 @@
+FROM ubuntu:20.04
+
+ENV TZ=UTC
+RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
+
+RUN apt-get update && apt-get -y install --no-install-recommends \
+        cmake \
+        virtualenv \
+        rapidjson-dev \
+        libfuse2 \
+        libssl-dev \
+        libboost-dev \
+        libxcb-randr0-dev \
+        libboost-system-dev \
+        libboost-filesystem-dev \
+        libpulse-dev \
+        libxkbcommon-x11-0 \
+        build-essential \
+        libgl1-mesa-dev \
+        libxcb-icccm4 \
+        libxcb-image0 \
+        libxcb-keysyms1 \
+        libxcb-render-util0 \
+        libxcb-xinerama0
+
+RUN apt-get -y install \
+        git \
+        lsb-release \
+        python3-pip && \
+        apt-get clean all
+
+# Install Qt as we do in CI
+
+RUN pip3 install -U pip && \
+        pip3 install aqtinstall && \
+        aqt install-qt linux desktop 5.12.12 && \
+        mkdir -p /opt/qt512 && \
+        mv /5.12.12/gcc_64/* /opt/qt512
+
+ADD . /src
+
+RUN mkdir /src/build
+
+# cmake
+RUN cd /src/build && \
+        CXXFLAGS=-fno-sized-deallocation cmake \
+        -DCMAKE_INSTALL_PREFIX=appdir/usr/ \
+        -DCMAKE_PREFIX_PATH=/opt/qt512/lib/cmake \
+        -DBUILD_WITH_QTKEYCHAIN=OFF \
+        ..
+
+# build
+RUN cd /src/build && \
+        make -j8