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/bionic_builder.dockerfile

@@ -0,0 +1,40 @@
+FROM ubuntu:bionic
+
+ENV LANG C.UTF-8
+
+RUN apt update -q
+RUN \
+  apt install -q -y --no-install-recommends \
+# Base build tools
+    build-essential automake libtool cmake ccache pkg-config autopoint patch \
+    python3-pip python3-setuptools python3-wheel git subversion wget unzip \
+    ninja-build \
+# Packaged dependencies
+    libbz2-dev libmagic-dev uuid-dev zlib1g-dev \
+    libmicrohttpd-dev aria2 \
+# Qt packages
+    libqt5gui5 qtbase5-dev qtwebengine5-dev libqt5svg5-dev qt5-image-formats-plugins qt5-default \
+# To create the appimage of kiwix-desktop
+    libfuse2 fuse patchelf \
+# Flatpak tools
+    elfutils flatpak flatpak-builder \
+# Other tools (to remove)
+#    vim less grep \
+  && \
+  apt-get clean -y && \
+  rm -rf /usr/share/doc/* /var/cache/debconf/*
+
+# Create user
+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=ci_builder:ci_builder . kiwix-build
+RUN pip3 install --user -e ./kiwix-build
+
+ENV TRAVIS_BUILD_DIR /home/ci_builder/kiwix-build
+ENV TRAVIS_OS_NAME linux_bionic
+
+CMD kiwix-build/travis/compile_all.py