🗂️ refactor: organize Dockerfiles into docker/ directory and update all references

adr/0020-docker-build-strategy.md

@@ -126,7 +126,7 @@ EXPOSE 8080
 ENTRYPOINT ["/app/dance-lessons-coach"]
 ```
 
-**Dockerfile** - Development Dockerfile (kept for local development):
+**docker/Dockerfile** - Development Dockerfile (kept for local development):
 ```dockerfile
 # Multi-stage build for development
 FROM golang:1.26.1-alpine AS builder
@@ -147,6 +147,16 @@ EXPOSE 8080
 ENTRYPOINT ["/app/dance-lessons-coach"]
 ```
 
+### File Organization
+
+All Dockerfiles are now organized in the `docker/` directory:
+- `docker/Dockerfile` - Development Dockerfile
+- `docker/Dockerfile.build` - Build cache Dockerfile
+- `docker/Dockerfile.prod` - Production Dockerfile (development only, uses latest)
+- `docker/Dockerfile.prod.template` - Template for reference
+
+This organization keeps the root directory clean and makes it clear which files are for development vs production.
+
 ## Benefits
 
 ### CI/CD Pipeline Benefits