As appimage and flatpak use same ubuntu version, merge dockerfiles.

- Use the same structure than xenial dockerfile.
- Do not try to deploy in the docker image as the deployement is made on
  the host.
- Use user `ci_builder` instead of `builder` as it make flatpak fail.

travis/xenial_builder.dockerfile

@@ -20,22 +20,23 @@ RUN \
 # Cross compile i586
     libc6-dev-i386 lib32stdc++6 gcc-multilib g++-multilib \
 # Other tools (to remove)
-    vim less grep && \
+#    vim less grep \
+  && \
   apt-get clean -y && \
   rm -rf /usr/share/doc/* /var/cache/debconf/*
 
 # Create user
-RUN useradd --create-home builder
-USER builder
-WORKDIR /home/builder
-ENV PATH="/home/builder/.local/bin:${PATH}"
+RUN useradd --create-home ci_builder
+USER ci_builder
+WORKDIR /home/ci_builder
+ENV PATH="/home/ci_builder/.local/bin:${PATH}"
 
 # Install kiwix-build
-COPY --chown=builder:builder . kiwix-build
+COPY --chown=ci_builder:ci_builder . kiwix-build
 RUN pip3 install --user -e ./kiwix-build
 
-ENV TRAVIS_BUILD_DIR /home/builder/kiwix-build
-ENV GRADLE_USER_HOME /home/builder
-ENV TRAVIS_OS_NAME   linux_xenial
+ENV TRAVIS_BUILD_DIR /home/ci_builder/kiwix-build
+ENV GRADLE_USER_HOME /home/ci_builder
+ENV TRAVIS_OS_NAME linux_xenial
 
 CMD kiwix-build/travis/compile_all.py