Added Dockerfile for Flatpak repo-server & Build instructions under docs/ dir

docs/Flatpak_Build.md

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+# PyBitmessage Linux flatpak instructions
+
+## Requirements
+First make sure you have `flatpak` and `flatpak-builder` installed. Follow the
+instructions for your distribution on [flathub](https://flatpak.org/setup/). The
+instructions there only cover the installation of `flatpak`, but 
+`flatpak-builder` should be the same.
+
+## Build and Install
+Once you have `flatpak` and `flatpak-builder` installed:
+
+Build and Install the Base App
+
+```
+git clone git://github.com/Bitmessage/PyBitmessage.git
+cd PyBitmessage/
+git submodule update --init --recursive
+flatpak-builder --install --install-deps-from=flathub --force-clean --state-dir=build/.flatpak-builder build/_baseApp packages/flatpak/org.bitmessage.BaseApp.json
+```
+This will install the base app to your local flatpak user repository, it 
+takes a while to compile because QT4 and PyQt4 have to be build, among others. But this is only required once.
+
+Now Build and Install PyBitmessage App
+
+```
+flatpak-builder --install --install-deps-from=flathub --force-clean --state-dir=build/.flatpak-builder build/_PyBit packages/flatpak/org.bitmessage.PyBitmessage.json
+```
+
+# Run
+When installation is done you can launch PyBitmessage via the **command line**:
+`flatpak run org.bitmessage.PyBitmessage`
+
+Flatpak also exports a `.desktop` file, so you should be able to find and launch
+PyBitmessage via the **application launcher** of your Desktop (Gnome, KDE, ...).
+
+# Export
+You can create a single file "bundle", which allows you to copy and install the
+PyBitmessage flatpak on other devices of the same architecture as the build machine.
+
+## Create a local flatpak repository
+```
+flatpak-builder --repo=build/_flatpak_repo --force-clean --state-dir=build/.flatpak-builder build/_PyBit packages/flatpak/org.bitmessage.PyBitmessage.json
+```
+This will create a local flatpak repository in `build/_flatpak_repo/`.
+
+## Hosting the repository
+
+Place the `_flatpak_repo/` repo directory in the same directory as the dockerfile i.e `packages/flatpak/dockerfile`
+
+Build and run the docker image
+
+```
+docker build -t repo-server:latest .
+docker run -d -p 5000:5000 repo-server
+```
+
+## Installing PyBitmessage from repo-server
+
+Add the repository
+```
+sudo flatpak remote-add --no-gpg-verify pybitmessage http://localhost:5000/repo
+```
+
+Install and Run the app
+```
+sudo flatpak install test org.bitmessage.PyBitmessage
+flatpak run org.bitmessage.PyBitmessage
+```
+
+## Create a bundle
+```
+flatpak build-bundle build/_flatpak_repo build/pybitmessage.flatpak org.bitmessage.PyBitmessage
+```
+This will create a `pybitmessage.flatpak` bundle file in the `build/` directory. 
+
+This bundle can be copied to other systems or installed locally:
+```
+flatpak install pybitmessage.flatpak
+```
+
+The application can be run using flatpak:
+```
+flatpak run org.bitmessage.PyBitmessage
+```
+
+It can then be uninstalled with this command:
+```
+flatpak uninstall org.bitmessage.PyBitmessage
+```
+
+This way of building an application is very convenient when preparing flatpaks
+for testing on another system of the same processor architecture.
+
+## Cleanup
+If you want to free up disk space you can remove the `Sdk` runtime again:
+`flatpak uninstall org.freedesktop.Sdk//19.08`
+
+You can also delete the `build` directory again.

packages/flatpak/dockerfile

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+FROM ubuntu:20.04
+
+MAINTANER Your Name "kuldeep.k@cisinlabs.com"
+
+RUN apt-get update -y && \
+    apt-get install -y python-pip python-dev
+
+# We copy just the requirements.txt first to leverage Docker cache
+COPY ./requirements.txt /app/requirements.txt
+
+WORKDIR /app
+
+RUN pip install -r requirements.txt
+
+COPY . /app
+
+ENTRYPOINT [ "python" ]
+
+CMD [ "server.py" ]

packages/flatpak/requirements.txt

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+flask

packages/flatpak/server.py

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+from flask import Flask, send_from_directory
+
+app = Flask(__name__)
+
+@app.route("/repo/<path:path>")
+def static_dir(path):
+    return send_from_directory("repo", path)
+
+if __name__ == "__main__":
+    app.run(host="0.0.0.0", port=5000, debug=True)