Add possibility to use secrets in environment variables (#512)

README.md

@@ -277,6 +277,8 @@ Optionally you can also define a prefix to only match predefined enviroment vari
 
 `ENV_VARIABLE_PREFIX="CFG_"` <-- this is the default prefix
 
+If you want use file for value (like when use secrets) you can add suffix `_FILE` to your variable name (in  run command).
+
 There are some limitations to what characters you can use.
 
 | Type  | Allowed Characters  |
@@ -323,7 +325,7 @@ services:
       # and here are the actual variables
       CFG_DB_HOST: "http://localhost:3306"
       CFG_DB_NAME: "minecraft"
-      CFG_DB_PASSWORD: "ug23u3bg39o-ogADSs"
+      CFG_DB_PASSWORD_FILE: "/run/secrets/db_password"
     restart: always
   rcon:
     image: itzg/rcon
@@ -336,8 +338,16 @@ services:
 volumes:
   mc:
   rcon:
+
+secrets:
+  db_password:
+    file: ./db_password
 ```
 
+The content of `db_password`:
+
+    ug23u3bg39o-ogADSs
+
 ## Running a Bukkit/Spigot server
 
 Enable Bukkit/Spigot server mode by adding a `-e TYPE=BUKKIT -e VERSION=1.8` or `-e TYPE=SPIGOT -e VERSION=1.8` to your command-line.
@@ -470,7 +480,7 @@ in either persistent volumes or a downloadable archive.
 A [Tuinity](https://github.com/Spottedleaf/Tuinity) server, which is a fork of Paper aimed at improving server performance at high playercounts.
 
     -e TYPE=TUINITY
-    
+
 > **NOTE** only `VERSION=LATEST` is supported
 
 ## Running a Magma server
@@ -478,7 +488,7 @@ A [Tuinity](https://github.com/Spottedleaf/Tuinity) server, which is a fork of P
 A [Magma](https://magmafoundation.org/) server, which is a combination of Forge and PaperMC, can be used with
 
     -e TYPE=MAGMA
-    
+
 > **NOTE** there are limited base versions supported, so you will also need to  set `VERSION`, such as "1.12.2"
 
 ## Running a Server with a Feed-The-Beast (FTB) / CurseForge modpack