Dependabot or Renovate for go.mod updates

[bilby91]

Set up Dependabot (.github/dependabot.yml) for weekly go.mod + GitHub Actions updates. Dependencies that matter most:

• github.com/moby/moby/{api,client} — Docker SDK
• github.com/google/go-containerregistry — OCI feature pulls
• github.com/compose-spec/compose-go/v2 — compose parsing
• github.com/tidwall/jsonc — JSONC parser
• gopkg.in/yaml.v3

Plus the GitHub Actions versions to stay ahead of the Node.js 20 → 24 deprecation (#6).