diff --git a/.ci/Dockerfile-ubuntu-18.04 b/.ci/Dockerfile-ubuntu-18.04
index d635e83..bb07059 100644
--- a/.ci/Dockerfile-ubuntu-18.04
+++ b/.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