Use gomplate to template Dockerfiles

[Ilyes512]

Summary

• Add Dockerfile.tmpl as the single source of truth for all three variant Dockerfiles
• Add variants/{fpm,apache,frankenphp}.yaml with static per-variant metadata (label, doc URL, frankenphp flag) — base image tag is never stored here
• Add task generate (runs gomplate via Docker) and task install:hooks / task remove:hooks for the pre-commit hook
• Wire a template-check job into the PR workflow so build only runs after check confirms the committed Dockerfiles match the template output

Test plan

• task generate produces no diff against the committed Dockerfiles
• Edit Dockerfile.tmpl, run task generate, confirm change appears in all three Dockerfiles
• CI check job passes on this PR