docker: configuration improvements

* Improves Docker configuration to speed up the build process by
  profiting from cache, to use the current user identity instead of
  root, and to run the simple example application as is done in
  `docker-compose`.

  Example session:

    $ docker-compose build
    $ docker-compose up
    $ firefox http://0.0.0.0:5000/
    $ docker-compose run web python setup.py test

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>

Dockerfile

@@ -7,9 +7,31 @@
 # it under the terms of the Revised BSD License; see LICENSE file for
 # more details.
 
+# Use Python-2.7:
 FROM python:2.7
-ADD . /code
+
+# Install some prerequisites ahead of `setup.py` in order to profit
+# from the docker build cache:
+RUN pip install 'coverage<4.0a1' \
+                flask \
+                pep257 \
+                pytest \
+                pytest-cov \
+                pytest-pep8 \
+                sphinx
+
+# Add sources to `code` and work there:
 WORKDIR /code
-RUN pip install pep257
-RUN pip install sphinx
-RUN python setup.py develop
+ADD . /code
+
+# Install Flask-Menu:
+RUN pip install -e .[docs]
+
+# Run container as user `flaskmenu` with UID `1000`, which should match
+# current host user in most situations:
+RUN adduser --uid 1000 --disabled-password --gecos '' flaskmenu && \
+    chown -R flaskmenu:flaskmenu /code
+
+# Start simple example application:
+USER flaskmenu
+CMD  ["python", "examples/simple/app.py"]