feat: publish both stable and validator-SNAPSHOT Docker images

[davidgamez]

Description

The API depends on the gtfs-validator core, which isn't fully released yet. This
adds an opt-in path to build against pre-release SNAPSHOTs while keeping stable as
the default — and publishes both as distinct Docker images.

Changes

• pom.xml — default stays on the stable validator core (8.0.1). New opt-in
  snapshot Maven profile (-Psnapshot) overrides gtfs-validator.version to
  8.0.2-SNAPSHOT and enables the public Maven Central snapshot repository.
  Version is overridable via -Dgtfs-validator.version=.
• Dockerfile — new MAVEN_PROFILES build arg, forwarded to the Maven build.
• .github/workflows/docker.yml — stable + snapshot build matrix. Image
  tags are derived from pom.xml so they never drift. PRs build both variants but
  push neither.
• README.md — documents the -Psnapshot build, the Docker build arg, and the
  image tag scheme.

Image tags

Variant	Tags
Stable	latest, 1.0.0-validator8.0.1
Snapshot	1.0.0-validator8.0.2-SNAPSHOT (never latest)

Tag format is <apiVersion>-validator<validatorCoreVersion> — the validator infix
scopes the trailing version to the validator core, since the API and core
versions evolve independently.

Verification

• Version derivation correct for both variants.
• Snapshot dependency resolves from Maven Central.
• End-to-end snapshot Docker build succeeds; container boots and /v2/metadata
  reports validatorVersion: 8.0.2-SNAPSHOT.