Make debugging actually work on linux by installing gdb. Also, add a manual image for testing the tests

tests/ci/image/Dockerfile

@@ -10,23 +10,29 @@ RUN apt-get update && \
                      libncurses5-dev libncursesw5-dev \
                      git \
                      tcl-dev \
-                     tcllib && \
+                     tcllib \
+                     gdb \
+                     lldb && \
   apt-get -y autoremove
 
 RUN ln -fs /usr/share/zoneinfo/Europe/London /etc/localtime && \
   dpkg-reconfigure --frontend noninteractive tzdata
 
+## cleanup of files from setup
+RUN rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+
 ARG VIM_VERSION=v8.1.1270
 
+ENV CONF_ARGS "--with-features=huge \
+               --enable-python3interp \
+               --enable-terminal \
+               --enable-multibyte \
+               --enable-fail-if-missing"
+
 RUN mkdir -p $HOME/vim && \
     cd $HOME/vim && \
     git clone https://github.com/vim/vim && \
     cd vim && \
     git checkout ${VIM_VERSION} && \
-    ./configure --with-features=huge \
-                --enable-python3interp \
-                --enable-terminal \
-                --enable-multibyte \
-                --enable-fail-if-missing && \
     make -j 4 install