Investigating AppImage version 2

Ciao.
In these days I was trying to investigate the possibility of creating a version 2 AppImage for the Linux distribution.
As already said before, this would give some benefits, among which the possibility of autoupdating the AppImage, #279707: Automatic updates on Linux
I tried from scratch inside an Ubuntu 14.04 64bit virtual image, with linuxdeploy tool https://github.com/linuxdeploy/linuxdeploy
Here is the log of what I did:
sudo apt-get update
sudo apt-get -y install git unzip libxslt1-dev libxrender-dev libxcomposite-dev libdrm-dev libgl1-mesa-dev libopenal1 gstreamer0.10-plugins-base software-properties-common
sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get -y install g++-4.9 wget make curl alsa libasound2-dev portaudio19-dev libportmidi-dev libsndfile1-dev zlib1g-dev libfreetype6-dev libfontconfig1-dev lame libmp3lame-dev libegl1-mesa-dev libpulse-dev libnss3-dev libxss1 libnspr4-dev libxi-dev libxcursor-dev libxtst-dev
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 40 --slave /usr/bin/g++ g++ /usr/bin/g++-4.9
mkdir qt5
wget -q -O qt5.zip http://utils.musescore.org.s3.amazonaws.com/qt593.zip
unzip -qq qt5.zip -d qt5
rm -f qt5.zip
CMAKE_URL="http://www.cmake.org/files/v3.5/cmake-3.5.1-Linux-x86_64.tar.gz"
mkdir cmake
wget --no-check-certificate --quiet -O - "${CMAKE_URL}" | tar --strip-components=1 -xz -C cmake
export PATH="${PWD}/qt5/bin:$PATH"
export QT_PLUGIN_PATH="${PWD}/qt5/plugins"
export QML2_IMPORT_PATH="${PWD}/qt5/qml"
export LD_LIBRARY_PATH="${PWD}/qt5/lib:$LD_LIBRARY_PATH"
export PATH="${PWD}/cmake/bin:${PATH}"

../linuxdeploy-x86_64.AppImage --appdir AppDir --output appimage -d AppDir/usr/share/applications/mscore.desktop -i AppDir/usr/share/icons/hicolor/scalable/apps/mscore.svg

The first block is used to prepare the environment for the compilation, which I copied straight from the Docker image used at the moment for MuseScore AppImage compilation.
First question: I see that since January the Docker image is based on Ubuntu 14.04; since also Travis gives an Ubuntu 14.04 machine, is the Docker file still necessary? Can't we create the AppImage directly inside Travis virtual machine?
(Previously, the Docker file was using Ubuntu 12.04, but I tried with an Ubuntu 12.04 virtual machine and got a series of errors)

The second block simply downloads the AppImage of linuxdeploy.
The third block fetches MuseScore source code.
The fourth block is the "make and make install" compilation core.
The fifth block are a couple of workarounds (already used in the current MuseScore AppImage) to avoid a crash at MuseScore AppImage startup.
The sixth block creates the AppImage itself.

As I can see, copying the "AppRun.sh", the "portable-utils", and the workaround for jack library before running linuxdeploy should create a working AppImage (see: https://github.com/probonopd/linuxdeployqt/wiki/Custom-wrapper-script-i… and linuxdeploy should avoid overwriting AppRun.sh https://github.com/linuxdeploy/linuxdeploy/commit/2f7ad201f1df3c9 - to be verified).
One of my concerns is the fact that with linuxdeploy the folders qt5/plugins and qt5/qml are not copied, but these can be manually copied before creating the AppImage.
Second question: Do we really need the whole contents of these folders? How can we understand what is really needed and what can be left out?
I know for sure that libqxcb.so is necessary otherwise the application won't start (and indeed the AppImage I managed to build runs only in the virtual machine in which it was created).

linuxdeploy creates in addition folder share/doc with the copyrights for the libraries it can find information on the system.

These are the libraries included in the current AppImage, but not copied by linuxdeploy in the new AppImage - are they maybe needed by Qt plugins?
libEGL.so.1
libopenal.so.1
libQt5Concurrent.so.5
libQt5DBus.so.5
libQt5Designer.so.5
libQt5Multimedia.so.5
libQt5MultimediaWidgets.so.5
libQt5OpenGL.so.5
libQt5Sensors.so.5
libQt5WebEngine.so.5
libQt5XcbQpa.so.5

These are additional libraries copied by linuxdeploy in the new AppImage, but not present in the current AppImage:
libasyncns.so.0
libdbus-1.so.3
libffi.so.6
libFLAC.so.8
libgbm.so.1
libgmodule-2.0.so.0
libgthread-2.0.so.0
libjson-c.so.2
libnsl.so.1
libogg.so.0
libpcre.so.3
libpulsecommon-4.0.so
libpulse.so.0
libsndfile.so.1
libvorbisenc.so.2
libvorbisfile.so.3
libvorbis.so.0
libwayland-client.so.0
libwayland-server.so.0
libwrap.so.0
libX11-xcb.so.1
libXau.so.6
libxcb-glx.so.0
libxcb-present.so.0
libxcb-sync.so.1
libxcb-xfixes.so.0
libXcomposite.so.1
libXcursor.so.1
libXdamage.so.1
libXdmcp.so.6
libXext.so.6
libXfixes.so.3
libXi.so.6
libXrandr.so.2
libXrender.so.1
libxshmfence.so.1
libXtst.so.6
libXxf86vm.so.1

I don't fully understand the chain (Travis + Docker + a series of custom scripts) used to build the current AppImage, so I don't think I could be able to write a PR for an AppImage version 2, but at least with linuxdeploy the work seems to be easier.