Added support for ubuntu 18.04

docker_builds/ubuntu_bionic/Dockerfile

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+FROM ubuntu:bionic
+ENV DEBIAN_FRONTEND=noninteractive 
+COPY do_build.sh /root
+RUN chmod +x /root/do_build.sh

docker_builds/ubuntu_bionic/do_build.sh

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+#!/bin/bash
+set -e
+cd /root
+
+# Update repos to get a more recent cmake version
+apt update
+apt install -y gpg wget
+wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /usr/share/keyrings/kitware-archive-keyring.gpg >/dev/null
+echo 'deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ bionic main' | tee /etc/apt/sources.list.d/kitware.list >/dev/null
+apt update
+
+# Install dependencies and tools
+apt install -y build-essential cmake git libfftw3-dev libglfw3-dev libglew-dev libvolk1-dev libsoapysdr-dev libairspyhf-dev libairspy-dev \
+            libiio-dev libad9361-dev librtaudio-dev libhackrf-dev librtlsdr-dev libbladerf-dev liblimesuite-dev p7zip-full wget portaudio19-dev
+
+# Install SDRPlay libraries
+wget https://www.sdrplay.com/software/SDRplay_RSP_API-Linux-3.07.1.run
+7z x ./SDRplay_RSP_API-Linux-3.07.1.run
+7z x ./SDRplay_RSP_API-Linux-3.07.1
+cp x86_64/libsdrplay_api.so.3.07 /usr/lib/libsdrplay_api.so
+cp inc/* /usr/include/
+
+cd SDRPlusPlus
+mkdir build
+cd build
+cmake .. -DOPT_BUILD_SDRPLAY_SOURCE=ON -DOPT_BUILD_BLADERF_SOURCE=OFF -DOPT_BUILD_LIMESDR_SOURCE=ON -DOPT_BUILD_NEW_PORTAUDIO_SINK=ON -DOPT_OVERRIDE_STD_FILESYSTEM=ON
+make -j2
+
+cd ..
+sh make_debian_package.sh ./build libfftw3-dev libglfw3-dev libglew-dev libvolk1-dev