doc: Add docker secret as option for OpenVPN credentials in FAQ

Update docs/faq.md

Co-authored-by: Aemulation <[email protected]>

docs/faq.md

@@ -168,11 +168,14 @@ if basic network connectivity is broken. You can write your own script and add i
 
 This container has the `autoheal` label by default so it is compatible with the [willfarrell/autoheal image](https://hub.docker.com/r/willfarrell/autoheal/)
 
+
 ## Send Username and Password via a file
 
-Depending on your setup, you may not want to send your VPN user/pass via environment variables (the main reason being, it is accessible via docker inspect). If you prefer, there is a way to configure the container to use a file instead. 
+Depending on your setup, you may not want to send your VPN user/pass via environment variables (the main reason being, it is accessible via docker inspect). If you prefer, there are two methods of avoiding credentials to be set via environment variables:
+ - Option 1: Configure the container to use a file. 
+ - Option 2: Configure the container to use a `secret`. (Only available for Docker Swarm)
 
-*Procedure*
+*Procedure for Option 1: Configuring the container to use a file*
 1. Create a text file with a username and password in it, each on a separate line.
 For this example, we will assume it is located at `./openvpn-credentials.txt`
 ```
@@ -226,6 +229,52 @@ services:
  image: haugene/transmission-openvpn
 ```
 
+*Procedure for Option 2: Configure the container to use a `secret`*
+
+Note: [Docker Secrets](https://docs.docker.com/engine/swarm/secrets/) are only available for Docker Swarm (`docker-compose`). If you run the container standalone, refer to Option 1.
+
+1. Create a text file with a username and password in it, each on a separate line.
+For this example, we will assume it is located at `./openvpn-credentials.txt`
+```
+this_is_my_username
+this_is_my_password
+```
+
+1. Mount the file as a secret like below. *The name of the secret must be exactly `openvpn_creds`, which will be exposed to the container at `/run/secrets/openvpn_creds`.
+
+The example docker-compose.yml looks like this:
+
+```
+version: '3.3'
+secrets:
+ openvpn_creds:
+ file: './openvpn-credentials.txt'
+services:
+ transmission-openvpn:
+ cap_add:
+ - NET_ADMIN
+ volumes:
+ - '/your/storage/path/:/data'
+ environment:
+ - OPENVPN_PROVIDER=PIA
+ - OPENVPN_CONFIG=france
+ - OPENVPN_USERNAME=**None**
+ - OPENVPN_PASSWORD=**None**
+ - LOCAL_NETWORK=192.168.0.0/16
+ logging:
+ driver: json-file
+ options:
+ max-size: 10m
+ secrets:
+ - openvpn_creds
+ ports:
+ - '9091:9091'
+ image: haugene/transmission-openvpn
+```
+
+Bonus tip: The same steps can be followed for `rpc_creds`, as shown in [[How do I enable authentication in the web ui](#how_do_i_enable_authentication_in_the_web_UI)].
+
+
 ## AUTH: Received control message: AUTH_FAILED
 
 If your logs end like this, the wrong username/password was sent to your VPN provider.