Add docs for connecting mathesar to local db

docs/docs/install/docker-compose/connecting-local-db.md

@@ -0,0 +1,94 @@
+# Connecting Mathesar to a host database
+
+## Prerequisites
+
+1. Locate `postgresql.conf` & `pg_hba.conf` file on the host machine. This can be located using `psql` shell by executing the following respectively.
+
+    ```
+    SHOW config_file;
+    ```
+
+    and
+
+    ```
+    SHOW hba_file;
+    ```
+
+2. Find the appropriate IP addresses of the `docker0` interface and the `mathesar_default` interface. This can be found by exectuting the following in the host's terminal.
+
+    ```
+    docker network inspect -f "docker0 IP: {{range .IPAM.Config}}{{.Gateway}}{{end}}" bridge && docker network inspect -f "mathesar_default IP: {{range .IPAM.Config}}{{.Subnet}}{{end}}" mathesar_default
+    ```
+
+3. Locate the `docker-compose.yml` & `.env` file on the host machine, by default it is located at `/etc/mathesar`.
+
+4. Stop Mathesar if already running:
+
+    === "Linux"
+        ```
+        sudo docker compose -f /etc/mathesar/docker-compose.yml --profile prod down
+        ```
+
+    === "MacOS"
+        ```
+        docker compose -f /etc/mathesar/docker-compose.yml --profile prod down
+        ```
+
+
+## Steps
+
+1. Edit the `postgresql.conf` file and add the IP of `docker0` interface in the `listen_addresses` setting. Uncomment this line if it's conmmented out.
+
+    ```
+    listen_addresses = 'localhost, <your-docker0-ip>'
+    ```
+
+2. Modify the `pg_hba.conf` file and grant access to the `mathesar_default` interface. Add the following line at the bottom of the file:
+
+    ```
+    host    all             all             <your-mathesar_default-ip>           scram-sha-256
+    ```
+
+3. Add an extra host for the prod container in the `docker-compose.yml` file:
+
+    ```
+    extra_hosts:
+          - "host.docker.internal:<your-docker0-ip>"
+    ```
+
+4. Set the appropriate environment variables in the `.env` file to establish the connection to the host database. Replace the `MATHESAR_DATABASES` env variable to the following:
+
+    ```
+    MATHESAR_DATABASES=(mathesar_tables|postgresql://<user_name>:<password>@host.docker.internal:<port-no>/<host_db_name>)
+    ```
+
+    If the host db is running on port 5432 you would need to add the following in the `.env` file:
+
+    ```
+    POSTGRES_PORT=<unused-port-on-host(e.g. 5555)> 
+    ```
+
+5. Restart postgres:
+
+    === "Linux"
+        ```
+        sudo systemctl restart postgresql
+        ```
+    === "MacOS"
+        ```
+        sudo brew services restart postgresql
+        ```
+
+6. Start Mathesar:
+
+    === "Linux"
+        ```
+        sudo docker compose -f /etc/mathesar/docker-compose.yml --profile prod up -d
+        ```
+
+    === "MacOS"
+        ```
+        docker compose -f /etc/mathesar/docker-compose.yml --profile prod up -d
+        ```
+
+You should have a successful connection to the host database now!

docs/docs/install/docker-compose/index.md

@@ -38,6 +38,9 @@
 
 1. When finished, the installer will display the URL where you can run Mathesar from your web browser.
 
+!!! note "Connecting to a local database"
+    Once you have successfully installed Mathesar, if you wish to connect it to an existing database that is running locally on the host machine, you can refer to the following [guide](./connecting-local-db.md).
+
 !!! info "Getting help"
     If you run into any problems during installation, see [troubleshooting](./troubleshooting.md) or [open a ticket describing your problem](https://github.com/centerofci/mathesar/issues/new/choose).