this patch introduce a docker file and docker compose example
so that developers can easily get a production docker based env
working quickly.

this by no means is a replacement to the installer or should
effect users besides the porpose of getting up an environment quickly

this does not try to solve docker problems for plugins (but it
can be used to build customized images with a custom plugin installed).

to test this pr you would need docker and docker-compose (I'm running docker-compose-1.22.0-2.)

to use this PR:
(install docker-compose)

docker-compose run app bundle exec rake db:create db:migrate db:seed
docker-compose up

in order to build your own docker image you can either use a service
(such as quay.io) tiggered by a git push or manually OR
to install a recent version of docker that supports multi-stage builds

in theory podman also supports it, but i had stability issues with it
while trying to build images.