Install tools and dependencies required for Flatpak build

Ubuntu 16.04 doesn't ship Flatpak in default repositories, so we need
to install it from the project's PPA. Additionally, it ships too
old Python for a tool used to create and upload Flathub builds to work,
so we need another PPA to install Python 3.6.

dockerbuild/Dockerfile

@@ -16,7 +16,19 @@ RUN apt-get -qq update && apt-get -qq dist-upgrade && \
   libsqlcipher-dev && \
   # git-lfs
   git lfs install && \
-  apt-get purge -y --auto-remove && rm -rf /var/lib/apt/lists/*
+  # used by flathub client \
+  apt-get install -y software-properties-common && \
+  add-apt-repository ppa:flatpak/stable && \
+  add-apt-repository ppa:deadsnakes/ppa && \
+  apt-get -qq update && \
+  apt-get -qq install -y --no-install-recommends flatpak ostree && \
+  apt-get -qq install -y python3.6 python3.6-dev libgirepository1.0-dev gir1.2-ostree-1.0 libcairo2-dev && \
+  curl -sSL https://bootstrap.pypa.io/get-pip.py -o get-pip.py && \
+  python3.6 get-pip.py && \
+  curl -sSL https://raw.githubusercontent.com/flatpak/flat-manager/master/flat-manager-client -o /usr/local/bin/flat-manager-client && \
+  chmod +x /usr/local/bin/flat-manager-client && \
+  /usr/local/bin/pip install aiohttp tenacity PyGObject && \
+  apt-get purge -y --auto-remove && rm -rf /var/lib/apt/lists/
 
 WORKDIR /project