Add Renovate config for automated p2pool version bumps

[sethforprivacy]

Summary

Sets up automated dependency updates via Renovate, modeled on the gold-standard config in sethforprivacy/simple-monerod-docker.

Changes

1. .github/renovate.json — same extends preset and customManager regex as the monerod repo. The custom manager already matches ARG ..._BRANCH=<value> lines, so it tracks P2POOL_BRANCH directly. With docker:enableMajor, the built-in docker manager will also keep the pinned ubuntu base image current.

2. Dockerfile annotation — adds

   # renovate: datasource=github-releases depName=SChernykh/p2pool
   

   immediately above ARG P2POOL_BRANCH=v4.17. This tells the custom manager to look up new p2pool releases from GitHub.

Validation

Validated with renovate-config-validator (renovate@latest):

INFO: Config validated successfully against 1 file(s)

Note: managerFilePatterns is the current field name (renamed from fileMatch) and requires Renovate ≥ 39. An older cached renovate@37 validator rejects it, but the hosted Mend Renovate app on this owner runs a current version — this is identical to the field used in the live monerod config.

Rollout

The hosted Mend Renovate app is already active on this owner's repos and will pick this config up automatically once merged. No CI build is triggered by this PR's renovate.json (the test-build workflow filters on Dockerfile); the 1-line annotation does touch the Dockerfile and will trigger a no-op-equivalent test build.