[N8N-4265] Improve docker compose setup with postgres (#3369)

* persist postgres data using docker volumes

* persist n8n data using docker volumes instead of writing to the host filesystem

* wait for postgres to be actually ready before starting the server

* run web server and workers as separate containers

* create a new `withPostgresAndWorker` example

docker/compose/withPostgresAndWorker/README.md

@@ -0,0 +1,26 @@
+# n8n with PostgreSQL and Worker
+
+Starts n8n with PostgreSQL as database, and the Worker as a separate container.
+
+
+## Start
+
+To start n8n simply start docker-compose by executing the following
+command in the current folder.
+
+
+**IMPORTANT:** But before you do that change the default users and passwords in the [`.env`](.env) file!
+
+```
+docker-compose up -d
+```
+
+To stop it execute:
+
+```
+docker-compose stop
+```
+
+## Configuration
+
+The default name of the database, user and password for PostgreSQL can be changed in the [`.env`](.env) file in the current directory.