Add Ubuntu 18.04 image for testing older GCC (#547)

docker/build_images.sh

@@ -31,4 +31,5 @@ pushd "${REPO_ROOT}/docker" &>/dev/null
 docker build -t rust:linux-386 linux-386 --load "${EXTRA_ARGS[@]}"
 docker build -t rust:linux-arm64 linux-arm64 --load "${EXTRA_ARGS[@]}"
 docker build -t rust:linux-x86_64 linux-x86_64 --load "${EXTRA_ARGS[@]}"
+docker build -t ubuntu:18.04 ubuntu-18.04 --load "${EXTRA_ARGS[@]}"
 popd &>/dev/null

docker/ubuntu-18.04/Dockerfile

@@ -0,0 +1,34 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+# SPDX-License-Identifier: Apache-2.0 OR ISC
+
+FROM --platform=linux/amd64 ubuntu:18.04
+
+SHELL ["/bin/bash", "-c"]
+
+ARG GOPROXY=direct
+
+RUN apt-get update && \
+    apt-get install -y ca-certificates && \
+    apt-get install -y cmake curl sudo  && \
+    apt-get install -y --no-install-recommends gpg-agent software-properties-common && \
+    add-apt-repository --yes ppa:longsleep/golang-backports && \
+    add-apt-repository --yes --update ppa:ubuntu-toolchain-r/test && \
+    apt-add-repository --yes ppa:git-core/ppa && \
+    apt-get update && \
+    apt-get install --assume-yes --no-install-recommends build-essential git cmake golang-go clang gcc-4.8 gcc-5 gcc-6 && \
+    git config --global --add safe.directory '*' && \
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs > ./rustup.sh && \
+    chmod +x ./rustup.sh && \
+    ./rustup.sh -y && \
+    . "$HOME/.cargo/env" && \
+    rustup component add rustfmt clippy && \
+    rm ./rustup.sh && \
+    cargo install rust-script && \
+    apt-get autoremove --purge -y && \
+    apt-get clean && \
+    apt-get autoclean && \
+    rm -rf /var/lib/apt/lists/* && \
+    rm -rf /tmp/*
+
+ENV CC=gcc-6
+ENV CXX=g++-6