docker: force build with g++-8 for cpp17

charconv is included by cpp>8.1, so we need to build the daemon
with libstdc++-8-dev and avoid the usage of g++7. Also remove
the docker for ubuntu 16 as we don't build for this version anymore.

Change-Id: If5b62996d3b84891b03b7a799989ed57dfeec589

docker/Dockerfile_ubuntu_16.04

-FROM ubuntu:16.04
-
-RUN apt-get update && \
-    apt-get install -y -o Acquire::Retries=10 \
-        git \
-        autoconf \
-        automake \
-        autopoint \
-        cmake \
-        libtool \
-	libcanberra-gtk3-dev \
-        libdbus-1-dev \
-        libdbus-c++-dev \
-        libupnp-dev \
-        libgnutls28-dev \
-        libebook1.2-dev \
-        libclutter-gtk-1.0-dev \
-        libclutter-1.0-dev \
-        libglib2.0-dev \
-        libgtk-3-dev \
-        libnotify-dev \
-        qtbase5-dev \
-        qttools5-dev \
-        qttools5-dev-tools \
-        yasm \
-        autotools-dev \
-        gettext \
-        libpulse-dev \
-        libasound2-dev \
-        libexpat1-dev \
-        libyaml-cpp-dev \
-        libboost-dev \
-        libxext-dev \
-        libxfixes-dev \
-        libspeex-dev \
-        libspeexdsp-dev \
-        uuid-dev \
-        libavcodec-dev \
-        libavutil-dev \
-        libavformat-dev \
-        libswscale-dev \
-        libavdevice-dev \
-        libopus-dev \
-        libudev-dev \
-        libjsoncpp-dev \
-        libnatpmp-dev \
-        libappindicator3-dev \
-        libqrencode-dev \
-        libnm-glib-dev \
-        libwebkit2gtk-4.0-dev \
-        libcrypto++-dev \
-        libva-dev \
-        libvdpau-dev
-
-# Tests framework
-RUN apt-get install -y -o Acquire::Retries=10 \
-        libcppunit-dev \
-        sip-tester
-
-RUN apt-get clean

docker/Dockerfile_ubuntu_18.04

 FROM ubuntu:18.04
 
 RUN apt-get update && \
+    apt install -y software-properties-common
+
+RUN add-apt-repository universe && \
+    apt-get update && \
     apt-get install -y -o Acquire::Retries=10 \
+        g++-8 \
+        gcc-8 \
+        cpp-8 \
+        file \
+        make \
+        libc6-dev \
+        libstdc++-8-dev \
         git \
         autoconf \
         automake \
         autopoint \
         cmake \
-        libtool \
         libdbus-1-dev \
         libdbus-c++-dev \
         libupnp-dev \
@@ -28,7 +38,6 @@ RUN apt-get update && \
         gettext \
         libpulse-dev \
         libasound2-dev \
-        libexpat1-dev \
         libpcre3-dev \
         libyaml-cpp-dev \
         libboost-dev \
@@ -57,7 +66,13 @@ RUN apt-get update && \
         libssl-dev \
         libsndfile1-dev \
         libsecp256k1-dev \
-        libasio-dev
+        libasio-dev \
+        libexpat1 libexpat1-dev
+
+RUN update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 50
+RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 50
+
+RUN ls -la /usr/include/c++/8/charconv
 
 # Tests framework
 RUN apt-get install -y -o Acquire::Retries=10 \