Add Renovate config for automated p2pool version bumps

Adds .github/renovate.json based on the gold-standard config in
sethforprivacy/simple-monerod-docker, plus a `# renovate:` annotation
above ARG P2POOL_BRANCH so Renovate tracks the upstream p2pool release.

- renovate.json: same extends + customManager regex as monerod. The
  customManager already matches `ARG ..._BRANCH=<value>` patterns, so it
  picks up P2POOL_BRANCH directly. The docker manager (via
  docker:enableMajor) will also keep the pinned ubuntu base image current.
- Dockerfile: add
  `# renovate: datasource=github-releases depName=SChernykh/p2pool`
  immediately above `ARG P2POOL_BRANCH=v4.17`.

Validated with `renovate-config-validator` (renovate@latest):
"Config validated successfully". (Note: the field `managerFilePatterns`
requires Renovate >= 39; the hosted Mend app on this owner runs current
Renovate, matching the live monerod config.)

The hosted Mend Renovate app is already active on this owner's repos and
will pick this up automatically once merged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/renovate.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended",
+    "docker:enableMajor",
+    "mergeConfidence:all-badges",
+    ":disableRateLimiting",
+    ":semanticCommits"
+  ],
+  "rebaseWhen": "conflicted",
+  "customManagers": [
+    {
+      "customType": "regex",
+      "managerFilePatterns": [
+        "/(^|/)Dockerfile$/"
+      ],
+      "matchStrings": [
+        "# renovate: datasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?\\sARG .*?_VERSION=(?<currentValue>.*)(\\sARG .*?_CHECKSUM=(?<currentDigest>.*))?\\s",
+        "# renovate: datasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?\\sARG .*?_BRANCH=(?<currentValue>.*)(\\sARG .*?_COMMIT_HASH=(?<currentDigest>.*))?\\s"
+      ],
+      "versioningTemplate": "{{#if versioning}}{{versioning}}{{/if}}"
+    }
+  ]
+}

Dockerfile

@@ -1,3 +1,4 @@
+# renovate: datasource=github-releases depName=SChernykh/p2pool
 ARG P2POOL_BRANCH=v4.17
 
 # Select latest Ubuntu LTS for the build image base