Supplement docker secrets usage information

Docker secrets has a couple of characteristics that may cause baffling
errors where configuration looks correct but the pihole container cannot
read the secret. The UID and GID of the secret file on the host must be
set to the same value used by the pihole process in the container
(typically 1000).

Signed-off-by: Bradley G Smith <[email protected]>

Author: buckaroogeek

README.md

@@ -144,7 +144,7 @@ If this variable is not detected and you have not already set one via `pihole se
 | `FTLCONF_[SETTING]` | unset | As per documentation | Customize pihole.toml with settings described in the [API Documentation](https://docs.pi-hole.net/api).<br><br>Replace `.` with `_`, e.g for `dns.dnssec=true` use `FTLCONF_dns_dnssec: 'true'`.<br/>Array type configs should be delimited with `;`.|
 | `PIHOLE_UID` | `1000` | Number | Overrides image's default pihole user id to match a host user id.<br/>**IMPORTANT**: id must not already be in use inside the container!|
 | `PIHOLE_GID` | `1000` | Number | Overrides image's default pihole group id to match a host group id.<br/>**IMPORTANT**: id must not already be in use inside the container!|
-| `WEBPASSWORD_FILE` | unset| `<Docker secret file>` | Set an Admin password using [Docker secrets](https://docs.docker.com/engine/swarm/secrets/). If `FTLCONF_webserver_api_password` is set, `WEBPASSWORD_FILE` is ignored. If `FTLCONF_webserver_api_password` is empty, and `WEBPASSWORD_FILE` is set to a valid readable file, then `FTLCONF_webserver_api_password` will be set to the contents of `WEBPASSWORD_FILE`. |
+| `WEBPASSWORD_FILE` | unset| `<Docker secret file>` | Set an Admin password using Docker secrets with [Swarm](https://docs.docker.com/engine/swarm/secrets/) or [Compose](https://docs.docker.com/compose/how-tos/use-secrets/). If `FTLCONF_webserver_api_password` is set, `WEBPASSWORD_FILE` is ignored. If `FTLCONF_webserver_api_password` is empty, and `WEBPASSWORD_FILE` is set to a valid readable file, then `FTLCONF_webserver_api_password` will be set to the contents of `WEBPASSWORD_FILE`. See [WEBPASSWORD_FILE Example](https://docs.pi-hole.net/docker/configuration/#webpassword_file-example) for additional information.|
 
 ### Advanced Variables