Add Dockerfile for application and documentation

This commit adds a `Dockerfile` to the root of the project, accompanied
by a `docker-compose.yml.example` for simplified deployment. The
`Dockerfile` is agnostic to whether it will be the webserver, the
consumer, or if it is run for a one-off command (i.e. creation of a
superuser, migration of the database, document export, ...).

The containers entrypoint is the `scripts/docker-entrypoint.sh` script.
This script verifies that the required permissions are set, remaps the
default users and/or groups id if required and installs additional
languages if the user wishes to.

After initialization, it analyzes the command the user supplied:

  - If the command starts with a slash, it is expected that the user
    wants to execute a binary file and the command will be executed
    without further intervention. (Using `exec` to effectively replace
    the started shell-script and not have any reaping-issues.)

  - If the command does not start with a slash, the command will be
    passed directly to the `manage.py` script without further
    modification. (Again using `exec`.)

The default command is set to `--help`.

If the user wants to execute a command that is not meant for `manage.py`
but doesn't start with a slash, the Docker `--entrypoint` parameter can
be used to circumvent the mechanics of `docker-entrypoint.sh`.

Further information can be found in `docs/setup.rst` and in
`docs/migrating.rst`.

For additional convenience, a `Dockerfile` has been added to the `docs/`
directory which allows for easy building and serving of the
documentation. This is documented in `docs/requirements.rst`.

docs/requirements.rst

@@ -101,3 +101,16 @@ you'd like to generate your own docs locally, you'll need to:
     $ pip install sphinx
 
 and then cd into the ``docs`` directory and type ``make html``.
+
+If you are using Docker, you can use the following commands to build the
+documentation and run a webserver serving it on `port 8001`_:
+
+.. code:: bash
+
+    $ pwd
+    /path/to/paperless
+
+    $ docker build -t paperless:docs -f docs/Dockerfile .
+    $ docker run --rm -it -p "8001:8000" paperless:docs
+
+.. _port 8001: http://127.0.0.1:8001