feat: automated project version management and docker release

[davidgamez]

Description

Derive version from git tags + snapshot/release Docker publishing

Automates the API version so it always matches the release tag, and publishes
Docker images for both main snapshots and tagged releases. No more hand-editing
pom.xml per release.

Changes

• Git-driven version — adds maven-git-versioning-extension (.mvn/). A tag
  vX.Y.Z builds version X.Y.Z; any other build is 0.0.0-SNAPSHOT. pom.xml
  keeps a placeholder version and is never edited for releases.
• Publishing (docker.yml) — both variants (stable-core and validator-core
  -snapshot) build on every event; pushes happen on non-PR events only. latest
  is applied on releases, stable variant only.
• Fork-aware image label — org.opencontainers.image.source is set from the
  building repo (lower-cased github.repository) so forks link to their own
  package; sensible default for local builds.
• Docs — new Releasing and Using the published images sections; updated
  Versions and CI tables.

Docker versioning

Image tags follow <apiVersion>-validator<validatorCoreVersion>. The API version
comes from git (release tag or 0.0.0-SNAPSHOT), and the validator core version
comes from the build variant. The two are independent: a main build is an API
snapshot, which is not the same as the validator-core snapshot.

On merge to main (no release tag), the API version resolves to
0.0.0-SNAPSHOT, so both variants publish as API snapshots:

Variant	Tag published on merge to main
stable-core	0.0.0-SNAPSHOT-validator8.0.1
snapshot-core	0.0.0-SNAPSHOT-validator8.0.2-SNAPSHOT

latest is not moved by main merges — these are dev/preview images.

On release (vX.Y.Z tag / GitHub Release), the API version resolves to X.Y.Z:

Variant	Tag published on release
stable-core	X.Y.Z-validator8.0.1 + latest
snapshot-core	X.Y.Z-validator8.0.2-SNAPSHOT

On pull requests, both variants build (to validate the Dockerfile) but nothing
is published.

Verified

• Branch build → 0.0.0-SNAPSHOT; tag v1.2.3 → 1.2.3.
• End-to-end Docker build; source label honors the fork override.
• docker.yml is valid YAML.