Run all backend services with a single docker-compose.yml

Changes:

• fixed forwarding points in gateway
• changed frontend port to 3000, to prevent conflict with envoy at 8080
• moved protos from outside to backend/gateway/protos, so that it can run from the project directory
• moved backend/database/config.js from outside to backend/gateway/src/database.js and backend/app/src/database.js, so that it can run from the project directory
• moved backend/Dockerfile to backend/app/Dockerfile, so that it can build from the project directory
• added backend/gateway/Dockerfile
• added start script to backend/app/package.json and backend/gateway/package.json, so that they can run with npm start command
• updated backend/docker-compose.yml to run all backend services