nixos/traefik: add environmentFiles option

nixos/modules/services/web-servers/traefik.nix

···
     ''
   else
     cfg.staticConfigFile;
+
+  finalStaticConfigFile =
+    if cfg.environmentFiles == []
+    then staticConfigFile
+    else "/run/traefik/config.toml";
 in {
   options.services.traefik = {
     enable = mkEnableOption (lib.mdDoc "Traefik web server");
···
       type = types.package;
       description = lib.mdDoc "Traefik package to use.";
     };
+
+    environmentFiles = mkOption {
+      default = [];
+      type = types.listOf types.path;
+      example = [ "/run/secrets/traefik.env" ];
+      description = lib.mdDoc ''
+        Files to load as environment file. Environment variables from this file
+        will be substituted into the static configuration file using envsubst.
+      '';
+    };
   };
 
   config = mkIf cfg.enable {
···
       startLimitIntervalSec = 86400;
       startLimitBurst = 5;
       serviceConfig = {
-        ExecStart =
-          "${cfg.package}/bin/traefik --configfile=${staticConfigFile}";
+        EnvironmentFile = cfg.environmentFiles;
+        ExecStartPre = lib.optional (cfg.environmentFiles != [])
+          (pkgs.writeShellScript "pre-start" ''
+            umask 077
+            ${pkgs.envsubst}/bin/envsubst -i "${staticConfigFile}" > "${finalStaticConfigFile}"
+          '');
+        ExecStart = "${cfg.package}/bin/traefik --configfile=${finalStaticConfigFile}";
         Type = "simple";
         User = "traefik";
         Group = cfg.group;
···
         ProtectHome = true;
         ProtectSystem = "full";
         ReadWriteDirectories = cfg.dataDir;
+        RuntimeDirectory = "traefik";
       };
     };
 

nixos/tests/traefik.nix

···
             sendAnonymousUsage = false;
           };
 
-          entryPoints.web.address = ":80";
+          entryPoints.web.address = ":\${HTTP_PORT}";
 
           providers.docker.exposedByDefault = false;
         };
+        environmentFiles = [(pkgs.writeText "traefik.env" ''
+          HTTP_PORT=80
+        '')];
       };
 
       systemd.services.simplehttp = {