Add a basic Dockerfile

Together with the preceding commits that allow a degree of configuration
of OpenSpending through environment variables, this Dockerfile allows
the creation of a binary full-system image of a specific version of
OpenSpending using [docker][1].

Full documentation of the use of docker and Dockerfiles can be found [on
the docker website][2], specifically at the [Dockerfile reference][3],
but briefly, this Dockerfile allows the building of a complete image
with:

    IMAGE=$(docker build .)

The subsequent image can be run as follows (assuming suitable values are
provided for DATABASE_URL, SOLR_URL and BROKER_URL):

    docker run -e DATABASE_URL=... \
               -e SOLR_URL=... \
               -e BROKER_URL=... \
               -d \
               "$IMAGE"

[1]: http://docker.io