add Docker deployment with Alpine image, release config and health check

- Alpine multi-stage Dockerfile (131 MB image)
- Release overlays (bin/server, bin/migrate), env.sh, Release module
- Health check endpoint at GET /health
- Fly.io config with SQLite volume mount
- Fix hardcoded paths in optimizer.ex and variant_cache.ex to use
  Application.app_dir/2 (breaks in releases where Plug.Static serves
  from a different directory than CWD)
- strip_beams: true in release config
- Optimised .dockerignore and .gitignore for mockup variants

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,52 @@
+# Build artifacts
+_build/
+deps/
+.elixir_ls/
+
+# Git
+.git/
+.gitignore
+
+# Test and dev
+test/
+.formatter.exs
+.credo.exs
+.dialyzer_ignore.exs
+
+# Database files
+*.db
+*.db-wal
+*.db-shm
+
+# Docker
+Dockerfile*
+.dockerignore
+docker-compose*
+
+# IDE
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# CI/docs
+CLAUDE.md
+PROGRESS.md
+ROADMAP.md
+docs/
+
+# Generated image variants (regenerated on startup from DB)
+priv/static/image_cache/
+
+# Mockup variants, digested copies, and converted formats.
+# Only the source .webp files (no size suffix) should enter the build context —
+# phx.digest creates its own hash-suffixed copies inside the builder.
+priv/static/mockups/*-400*
+priv/static/mockups/*-800*
+priv/static/mockups/*-1200*
+priv/static/mockups/*-thumb*
+priv/static/mockups/*.avif
+priv/static/mockups/*.jpg
+priv/static/mockups/*.gz