Created Docker only setup (markdown)

Docker-only-setup.md

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+⚠️ The following guides aims at a completely fresh install of Invoice Ninja with any web server you want and any database supported. The is no migration scenario covered.
+
+This might be a suitable guide for you if
+
+* your web server is on your host system already and you don't need another one bundled with IN
+* your database is on your host or any other server and you don't need another database instance
+* you just need IN an no extra web server and/or database
+
+⚠️ Please make sure you have enough knowledge to connect your web server to the running Invoice Ninja instance.
+
+## Prerequisites
+
+ℹ️ Read the [prerequisites section](https://github.com/invoiceninja/dockerfiles/wiki/prerequisites) carefully as you need all of these information for a proper setup.
+
+## Run the instance
+
+To run the Invoice Ninja instance you need to issue this command:
+
+```shell
+docker run -d \
+  -v /var/invoiceninja/public:/var/app/public \
+  -v /var/invoiceninja/storage:/var/app/storage \
+  -e APP_URL='http://ninja.dev' \
+  -e APP_KEY='<INSERT THE GENERATED APPLICATION KEY HERE>' \
+  -e DB_HOST='localhost' \
+  -e DB_DATABASE='ninja' \
+  -e DB_USERNAME='ninja' \
+  -e DB_PASSWORD='ninja' \
+  -p '9000:9000' \
+  --name invoiceninja \
+  invoiceninja/invoiceninja:alpine-4
+```
+
+The command above starts an instance 
+
+* of Invoice Ninja 4
+* with [host-mounted folders](https://github.com/invoiceninja/dockerfiles/wiki/prerequisites#create-folders-for-data-persistence)
+* listening on port `9000` - that's important for your web server
+
+All configuration is passed via environment variables. Check the [variables available here](https://github.com/invoiceninja/dockerfiles/wiki/differences-version-4-and-5#environment-variables), depending on which version of Invoice Ninja you are going to run.
+