refactor: abstract docker network creation via custom options

modules/common/default.nix

@@ -1,5 +1,6 @@
 {
   imports = [
+    ./docker.nix
     ./monitoring.nix
   ];
 

modules/common/docker.nix

@@ -0,0 +1,38 @@
+{ config, lib, pkgs, ... }:
+
+{
+  options.homelab.dockerNetworks = lib.mkOption {
+    description = "Declarative Docker networks to create before containers start.";
+    default = {};
+    type = lib.types.attrsOf (lib.types.submodule {
+      options = {
+        requiredBy = lib.mkOption {
+          type = lib.types.listOf lib.types.str;
+          default = [];
+          description = "List of systemd services that require this network (e.g., docker-containerName.service).";
+        };
+      };
+    });
+  };
+
+  config = {
+    systemd.services = lib.mapAttrs' (networkName: cfg:
+      lib.nameValuePair "docker-${networkName}-create-network" {
+        description = "Create Docker network for ${networkName}";
+        requiredBy = cfg.requiredBy;
+        after = [ "network.target" "docker.service" ];
+        requires = [ "docker.service" ];
+        wantedBy = [ "multi-user.target" ];
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+          ExecStart = pkgs.writeShellScript "create-${networkName}-docker-network" ''
+            if ! ${pkgs.docker}/bin/docker network ls | grep -q ${networkName}; then
+              ${pkgs.docker}/bin/docker network create ${networkName}
+            fi
+          '';
+        };
+      }
+    ) config.homelab.dockerNetworks;
+  };
+}