refactor: move database to monorepo

backend/database/README.md

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+# Database docker image
+[![pipeline status](https://img.shields.io/gitlab/pipeline/openstapps/database.svg?style=flat-square)](https://gitlab.com/openstapps/database/commits/master)
+[![license)](https://img.shields.io/badge/license-GPL--3.0--only-lightgrey.svg)](https://www.gnu.org/licenses/gpl-3.0.en.html)
+
+The used database is Elasticsearch 5.6.x. This images is based on
+[elasticsearch:5.6](https://hub.docker.com/r/library/elasticsearch/) from the
+[offical Docker Library project](https://docs.docker.com/docker-hub/official_repos/) and uses openjdk:8-jre-alpine as a
+base image.
+
+Additionally the [ICU Analysis Plugin](https://www.elastic.co/guide/en/elasticsearch/plugins/5.6/analysis-icu.html) is
+installed to provide unicode collation which is used for sorting.
+
+## Usage for testing purposes:
+Run it with this command: `docker run -d -p 9200:9200 registry.gitlab.com/openstapps/database:master`
+
+Currently there is an issue with elasticsearch and java on linux machines that temporarly can be solved by running:
+`sudo sysctl -w vm.max_map_count=262144` on the host machine. On restart of the machine this parameter will reset.
+To set it permanently, you will have to add it to `/etc/sysctl.conf` on machines that run systemd as a init deamon.
+
+## For production use:
+Please set `vm.max_map_count=262144` permanently. It is explained [here](https://www.elastic.co/guide/en/elasticsearch/reference/5.6/docker.html#docker-cli-run-prod-mode)
+