Add centos8 and Ubuntu Xenial dockerfiles

docker/centos8.Dockerfile

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+FROM centos:8
+
+# install powertools to get ninja-build
+RUN dnf -y install dnf-plugins-core \
+  && dnf config-manager --set-enabled powertools \
+  && yum install --assumeyes epel-release \
+  && yum install --assumeyes \
+    cmake \
+    diffutils \
+    gcc-c++ \
+    git \
+    ncurses-devel \
+    ninja-build \
+    python3 \
+    openssl \
+    sudo
+
+RUN pip3 install pexpect
+
+RUN groupadd -g 1000 fishuser \
+  && useradd  -p $(openssl passwd -1 fish) -d /home/fishuser -m -u 1000 -g 1000 fishuser -G wheel \
+  && 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

docker/xenial.Dockerfile

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+FROM ubuntu:16.04
+
+ENV LANG C.UTF-8
+ENV LC_ALL C.UTF-8
+
+RUN apt-get update \
+  && apt-get -y install \
+    build-essential \
+    cmake \
+    gettext \
+    git \
+    libncurses5-dev \
+    locales \
+    ninja-build \
+    python3 \
+    python3-pip \
+    sudo \
+  && locale-gen en_US.UTF-8
+
+# The python3-pexpect package on Xenial doesn't allow delaybeforesend to be None.
+# Install pexpect with pip which is newer.
+RUN pip3 install pexpect
+
+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