feat: use DOCKER_IMAGE from .env in Makefile

- Update Makefile to read DOCKER_IMAGE from .env file with proper precedence
- Environment variables override .env file values
- Fallback to 'meds-app:latest' if not defined anywhere
- Add new Docker targets: docker-run, docker-push, docker-config
- Improve help output with environment variable documentation
- Update APP_NAME_CONFIGURATION.md to show Makefile usage examples

Variable precedence:
1. Environment variable (highest)
2. .env file (medium)
3. Default fallback (lowest)

docs/setup/APP_NAME_CONFIGURATION.md

@@ -25,7 +25,10 @@ make deploy
 export APP_NAME="DevMeds"
 bun run dev
 
-# For Docker build
+# For Docker build with Makefile
+DOCKER_IMAGE="myregistry.com/myapp:v1.0" make docker-build
+
+# For Docker build with manual command
 APP_NAME="CustomApp" docker build -t myapp .
 ```
 
@@ -34,12 +37,15 @@ APP_NAME="CustomApp" docker build -t myapp .
 ```bash
 # .env
 APP_NAME=MyCustomApp
+DOCKER_IMAGE=myregistry.com/mycustomapp:dev
 
 # .env.production
 APP_NAME=ProductionApp
+DOCKER_IMAGE=myregistry.com/productionapp:latest
 
 # .env.staging
 APP_NAME=StagingApp
+DOCKER_IMAGE=myregistry.com/stagingapp:staging
 ```
 
 ## Where APP_NAME is Used