Clarifying docs about database on the host machine.

README.md

@@ -21,6 +21,11 @@ docker-compose logs -f
 Some services may require additional configuration. Check for additional `README.md` files
 or comments in the `docker-compose.yml` files or `.env.dist` templates.
 
+## Using a database server on the host from the container
+
+You need to change your database configuration to be able to do that. Check 
+[this](_docs/access_database_on_host_from_docker.md) documentation.
+
 ## Is it working?
 
 Not every stack is tested to fully work.

_docs/access_database_on_host_from_docker.md

@@ -0,0 +1,39 @@
+# Using database on host machine from Docker
+
+
+## Database configuration
+
+### PostgreSQL
+You need to make PostgreSQL listen not only `localhost`, but also Docker's network interface.
+
+To do that you need to add host machine IP address in the Docker network (usually `172.17.0.1`) to the
+`postgresql.conf` by changing `listen_addresses` parameter:
+```ini
+# Listen local interface and also Docker's network
+listen_addresses = 'localhost,172.17.0.1'
+```
+
+Then you need to allow apps inside Docker containers to authenticate. That could be done by adding following line to the
+`pg_hba.conf`:
+```
+# Docker network
+host    all             all             172.17.0.0/12            md5
+```
+
+Do not forget to restart your PostgreSQL server. For PostgreSQL 12 and `main` cluster it could be usually done like that:
+
+```shell
+systemctl restart postgresql@12-main.service
+```
+
+### MySQL / MariaDB
+
+TBW
+
+## Application configuration inside Docker
+
+Stacks which allow to use external database back-end should also map `host.docker.internal` to the host machine
+address inside Docker's default network.
+
+So to connect to the database from the application inside the container you should use `host.docker.internal` as the
+database host/address.

synapse/README.md

@@ -10,7 +10,10 @@ docker-compose run synapse generate
 
 After that you can edit `./data/homeserver.yaml` according to your needs.
 
-If you want to use full-fledged PostgreSQL instead of SQLite, you can check [this documentation](https://github.com/matrix-org/synapse/blob/master/docs/postgres.md).
+If you want to use full-fledged PostgreSQL instead of SQLite, you can check
+[this documentation](https://github.com/matrix-org/synapse/blob/master/docs/postgres.md).
+
+To use PostgreSQL running on the host machine, use [this](../_docs/access_database_on_host_from_docker.md) configuration.
 
 # Running the server
 

synapse/docker-compose.yml

@@ -1,13 +1,22 @@
 version: '3.7'
 
+networks:
+  # You need to create this network manually first!
+  synapse:
+    external: true
+
 services:
   synapse:
     # https://hub.docker.com/r/matrixdotorg/synapse
     # https://github.com/matrix-org/synapse/tree/master/docker
     image: "matrixdotorg/synapse:${IMAGE_VERSION:-latest}"
     container_name: synapse
-    # This way we'll be able to access PostgreSQL on local machine using 'localhost' when it's listening only locally.
+    hostname: synapse
-    network_mode: host
+    extra_hosts:
+      - 'host.docker.internal:host-gateway'
+    networks:
+      - default
+      - synapse
     ports:
       - "${INTERFACE_EXT:-127.0.0.1}:${HTTP_PORT_EXT:-8008}:${HTTP_PORT_INT:-8008}/tcp"
     volumes: