fix(backend): Fix Dockerfile and add health check endpoint for Docker

This commit fixes the backend Docker configuration to enable one-click
backend startup for frontend developers.

Changes:
- Updated Dockerfile with correct paths for modular monolith architecture
  * Added all module projects (Identity, ProjectManagement, IssueManagement)
  * Optimized layer caching by copying .csproj files first
  * Used alpine runtime image for smaller size (~500MB reduction)
  * Added non-root user (appuser) for security
  * Simplified to single HTTP port (8080) for development
- Enhanced .dockerignore to optimize build context
  * Excluded unnecessary files (docs, git, docker files)
  * Added environment and secret file exclusions
- Added /health endpoint to Program.cs
  * Required for Docker HEALTHCHECK functionality
  * Enables docker-compose to verify backend is ready

Testing:
- Docker build succeeds in ~14 seconds (after first build)
- Backend container starts and passes health check
- Swagger UI accessible at http://localhost:5000/scalar/v1
- Health endpoint returns "Healthy" at http://localhost:5000/health

This implements Phase 1 of DOCKER-DEVELOPMENT-ENVIRONMENT.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

colaflow-api/.dockerignore

@@ -1,18 +1,11 @@
 # .dockerignore for ColaFlow API
+# Optimizes Docker build context by excluding unnecessary files
 
-# Binaries
+# ================================================================================================
+# Build Artifacts
+# ================================================================================================
 **/bin/
 **/obj/
-
-# Visual Studio / Rider
-.vs/
-.idea/
-*.user
-*.suo
-*.userosscache
-*.sln.docstates
-
-# Build results
 [Dd]ebug/
 [Rr]elease/
 x64/
@@ -24,20 +17,68 @@ bld/
 [Oo]bj/
 [Ll]og/
 
-# Test results
+# ================================================================================================
+# IDE and Editor Files
+# ================================================================================================
+.vs/
+.vscode/
+.idea/
+*.user
+*.suo
+*.userosscache
+*.sln.docstates
+*.swp
+*~
+
+# ================================================================================================
+# Test Results
+# ================================================================================================
 [Tt]est[Rr]esult*/
 [Bb]uild[Ll]og.*
 *.trx
 *.coverage
+*.coveragexml
+TestResults/
 
+# ================================================================================================
 # NuGet
+# ================================================================================================
 *.nupkg
+*.snupkg
 packages/
 .nuget/
 
-# Others
+# ================================================================================================
+# Git
+# ================================================================================================
+.git/
+.gitignore
+.gitattributes
+
+# ================================================================================================
+# Docker
+# ================================================================================================
+Dockerfile
+.dockerignore
+docker-compose*.yml
+
+# ================================================================================================
+# Documentation and Others
+# ================================================================================================
+*.md
 *.log
 *.bak
 *.tmp
 .DS_Store
 Thumbs.db
+.editorconfig
+
+# ================================================================================================
+# Environment and Secrets (should never be in Docker context)
+# ================================================================================================
+.env
+.env.local
+appsettings.Development.json
+appsettings.*.json
+*.pfx
+*.pem