travis: switch to ubuntu 20.04

contrib/ci/Dockerfiles/z3_base_ubuntu_20.04.Dockerfile

@@ -0,0 +1,32 @@
+FROM ubuntu:20.04
+
+RUN apt-get update && \
+    apt-get -y --no-install-recommends install \
+        cmake \
+        make \
+        ninja-build \
+        clang \
+        g++ \
+        curl \
+        doxygen \
+        default-jdk \
+        git \
+        graphviz \
+        python3 \
+        python3-setuptools \
+        sudo
+
+RUN curl -SL https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb --output packages-microsoft-prod.deb && \
+    dpkg -i packages-microsoft-prod.deb && \
+    apt-get update && \
+    apt-get -y --no-install-recommends install dotnet-sdk-2.1
+
+# Create `user` user for container with password `user`.  and give it
+# password-less sudo access
+RUN useradd -m user && \
+    echo user:user | chpasswd && \
+    cp /etc/sudoers /etc/sudoers.bak && \
+    echo 'user  ALL=(root) NOPASSWD: ALL' >> /etc/sudoers
+USER user
+WORKDIR /home/user
+#ENV ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-7/bin/llvm-symbolizer