Install g++ package explicitly on Ubuntu

Apparently, g++ is only installed because of the Boost dependency.
Make the g++ dependency explicit to avoid future package errors.

.ci/Dockerfile-ubuntu-18.04

@@ -4,6 +4,7 @@ ARG toolchain
 
 RUN apt-get update
 RUN apt-get install -y libboost-all-dev cmake git ninja-build re2c
+RUN if [ "${toolchain}" = "gcc" ]; then apt-get install -y g++; fi
 RUN if [ "${toolchain}" = "clang" ]; then apt-get install -y clang; fi
 
 VOLUME /app