Add a Dockerfile to test 32 bit builds

Run it with:

    ./docker/docker_run_tests.sh docker/focal-32bit.Dockerfile

docker/focal-32bit.Dockerfile

@@ -0,0 +1,36 @@
+FROM ubuntu:20.04
+
+ENV LANG C.UTF-8
+ENV LC_ALL C.UTF-8
+ENV CXXFLAGS="-m32 -Werror=address -Werror=return-type" CFLAGS="-m32"
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update \
+  && apt-get -y install \
+    build-essential \
+    cmake \
+    g++-multilib \
+    gettext \
+    git \
+    lib32ncurses5-dev \
+    locales \
+    ninja-build \
+    pkg-config \
+    python3 \
+    python3-pexpect \
+    sudo \
+  && locale-gen en_US.UTF-8
+
+RUN groupadd -g 1000 fishuser \
+  && useradd -p $(openssl passwd -1 fish) -d /home/fishuser -m -u 1000 -g 1000 fishuser \
+  && adduser fishuser sudo \
+  && mkdir -p /home/fishuser/fish-build \
+  && mkdir /fish-source \
+  && chown -R fishuser:fishuser /home/fishuser /fish-source
+
+USER fishuser
+WORKDIR /home/fishuser
+
+COPY fish_run_tests.sh /
+
+CMD /fish_run_tests.sh