chore: improve renovate dependency detection

- Add renovate.json with custom regexManagers for:
  - Quoted FROM images (debian:buster)
  - SABNZBD_VERSION, TINI_VERSION, PAR2CMDLINE_VERSION env pins
- Disable gitlabci manager (legacy CI, migrated to Gitea Actions)
- Remove unnecessary quotes from FROM image reference

renovate.json

@@ -0,0 +1,47 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "gitlabci": {
+    "enabled": false
+  },
+  "customManagers": [
+    {
+      "customType": "regex",
+      "description": "Detect FROM images with optional quotes",
+      "fileMatch": ["^Dockerfile$"],
+      "matchStrings": [
+        "FROM\\s+\"?(?<depName>[^\":\\s]+):(?<currentValue>[^\"\\s]+)\"?"
+      ],
+      "datasourceTemplate": "docker"
+    },
+    {
+      "customType": "regex",
+      "description": "Detect SABnzbd release version pin",
+      "fileMatch": ["^Dockerfile$"],
+      "matchStrings": [
+        "SABNZBD_VERSION=(?<currentValue>[0-9.]+)"
+      ],
+      "depNameTemplate": "sabnzbd/sabnzbd",
+      "datasourceTemplate": "github-releases"
+    },
+    {
+      "customType": "regex",
+      "description": "Detect Tini release version pin",
+      "fileMatch": ["^Dockerfile$"],
+      "matchStrings": [
+        "TINI_VERSION\\s+v?(?<currentValue>[0-9.]+)"
+      ],
+      "depNameTemplate": "krallin/tini",
+      "datasourceTemplate": "github-releases"
+    },
+    {
+      "customType": "regex",
+      "description": "Detect par2cmdline release version pin",
+      "fileMatch": ["^Dockerfile$"],
+      "matchStrings": [
+        "PAR2CMDLINE_VERSION=v?(?<currentValue>[0-9a-z.]+-mt\\d+)"
+      ],
+      "depNameTemplate": "jkansanen/par2cmdline-mt",
+      "datasourceTemplate": "github-tags"
+    }
+  ]
+}