Add a centos7 Dockerfile

This tests building and running on centos7 with gcc 4.8.
To run it:

    ./docker/docker_run_tests.sh ./docker/centos7.Dockerfile

docker/centos7.Dockerfile

@@ -0,0 +1,28 @@
+FROM centos:7
+
+# install epel first to get cmake3
+RUN yum install --assumeyes epel-release https://repo.ius.io/ius-release-el7.rpm \
+  && yum install --assumeyes \
+    cmake3 \
+    gcc-c++ \
+    git224-core \
+    ncurses-devel \
+    ninja-build \
+    python3
+
+# cmake is called "cmake3" on centos7.
+RUN ln -s /usr/bin/cmake3 /usr/bin/cmake \
+  && pip3 install pexpect
+
+RUN groupadd -g 1000 fishuser \
+  && useradd -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