feat: upgrade to commitlint v21 on node 24

[robinbowes]

Fork of wagoid/commitlint-github-action, brought up to commitlint v21.

Why fork

Upstream is unmaintained — last commit to master was 2025-01-14, and the maintainer has no public activity. PR #837 bumps @commitlint to v20 and has been open and MERGEABLE since 2026-07-20 with no response.

The bug that matters to us: @commitlint/ensure v20 added an exemption for long lines containing a URL.

const URL_REGEX = /\bhttps?:\/\/\S+/;
value.split(/\r?\n/).every((line) => URL_REGEX.test(line) || ensure(line, max));

Pinned to ^19, this action fails Dependabot commits on body-max-line-length — their bodies are markdown link lists (one real example is 314 chars on a line) — while a local pre-commit hook resolving v21 passes the same commit.

What changed

• All @commitlint/* packages to ^21.
• Node 20.16.0 → 24.19.0 LTS in the Dockerfile, .nvmrc, engines, and workflows. commitlint v21 requires node >= 22.12, which is why upstream PR fix(deps): bump @commitlint packages to v20 wagoid/commitlint-github-action#837 stopped at v20.
• commitlint-plugin-function-rules ^4 → ^5.0.1. v4's peer range is @commitlint/lint ">=19 <20", which breaks npm ci outright; ^5.0.1 allows ">=19 <22".
• Release moved from Docker Hub (commit-and-tag-version + DOCKER_USERNAME/DOCKER_TOKEN) to ghcr.io, triggered by pushing a v* tag.
• CI gained an image-builds job. That job is what surfaced the peer conflict — npm install had silently resolved around it, only npm ci in the container failed.
• Fixed the FromAsCasing Dockerfile warning.

Verification

• npm run lint clean, 39/39 tests pass, npm run build succeeds.
• docker build succeeds; image reports v24.19.0 and @commitlint/lint 21.2.0.
• Replayed the action's own load+lint path (src/action.mjs:146) against a real commitlint.config.mjs and the Dependabot commit that broke CI:

Case	Result
Dependabot commit, 314-char body line	valid ✅
Sentence-case subject with subject-case: [0]	valid ✅ — the .mjs override is honored, so #836 does not bite us
Malformed commit	rejected ✅

After merge

1. Push tag v6.3.0 → publish-image.yml builds ghcr.io/yo61/commitlint-github-action:6.3.0.
2. The GHCR package must be set public, or consuming repos cannot pull it.
3. Point the 9 repos at this action by SHA.