feat: add OCI image labels to all Dockerfiles

[josecarlospeer-cloud]

Closes #101 (OCI labels item)

What

Replaces the bare LABEL maintainer=... with the standard org.opencontainers.image.* label set across all PHP versions.

Labels added

Label	Value
authors	Jorge Arco <jorge.arcoma@gmail.com>
source	https://github.com/jorge07/alpine-php
title	jorge07/alpine-php
description	Lightweight PHP-FPM Docker images based on Alpine Linux
licenses	Apache-2.0
url	https://hub.docker.com/r/jorge07/alpine-php
created	build-time (via BUILD_DATE ARG)
revision	build-time (via VCS_REF ARG)

Makefile

Added OCI_LABELS variable that computes BUILD_DATE and VCS_REF from the current date and git SHA, and passes them as --build-arg to all build and release targets.

Verification

$ docker inspect jorge07/alpine-php:8.3-oci-test --format '{{json .Config.Labels}}'
{
    "org.opencontainers.image.authors": "Jorge Arco <jorge.arcoma@gmail.com>",
    "org.opencontainers.image.created": "2026-02-22T08:30:09Z",
    "org.opencontainers.image.description": "Lightweight PHP-FPM Docker images based on Alpine Linux",
    "org.opencontainers.image.licenses": "Apache-2.0",
    "org.opencontainers.image.revision": "b7ac062",
    "org.opencontainers.image.source": "https://github.com/jorge07/alpine-php",
    "org.opencontainers.image.title": "jorge07/alpine-php",
    "org.opencontainers.image.url": "https://hub.docker.com/r/jorge07/alpine-php"
}  ✅