Add Docker setup, .env.example, updated .gitignore/CHANGELOG and ADRs 004-007 + arc42 template

.gitignore

@@ -1,2 +1,91 @@
-# ignore agentic files
-.github/instructions/
+# =============================================
+# Linear Void .gitignore
+# =============================================
+
+# Environment & Secrets
+.env
+.env.local
+.env.*.local
+
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+
+# Runtime data
+pids
+*.pid
+*.seed
+*.pid.lock
+
+# Dependency directories
+node_modules/
+jspm_packages/
+
+# Build outputs
+dist/
+build/
+out/
+*.min.js
+
+# Go build artifacts
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# FFmpeg temporary files
+*.ts.tmp
+*.m3u8.tmp
+hls/
+temp/
+
+# Database & local data
+*.db
+data/
+*.sqlite
+*.sqlite3
+
+# Docker
+.dockerignore
+**/.dockerignore
+
+# Editor & IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# OS generated files
+.DS_Store
+.DS_Store?
+._*
+.Spotlight-V100
+.Trashes
+ehthumbs.db
+Thumbs.db
+
+# Terraform / IaC state
+*.tfstate
+*.tfstate.backup
+.terraform/
+
+# Monitoring & temp
+prometheus/
+grafana/
+
+# Agent / Tool files
+.github/instructions/TODO.md
+.github/instructions/DONE.md
+# (keep AGENTS.md and other instruction files)
+
+# Temporary architecture artifacts
+**/*.tmp
+**/*.bak
+
+# Note: .github/instructions/ is partially ignored but AGENTS.md and core docs are committed