How to fix ERROR: for web Cannot create container for service web: invalid port specification in Docker

# WRONG: May be misinterpreted by YAML parser
services:
  web:
    ports:
      - 8080:80
      - 3000:3000
      - 8025:25    # This becomes 4825 due to base-60 parsing!

# CORRECT: Always quote port mappings
services:
  web:
    ports:
      - "8080:80"
      - "3000:3000"
      - "8025:25"

# Check your docker-compose.yml
services:
  web:
    ports:
      - "${WEB_PORT}:3000"  # WEB_PORT might be undefined

# .env
WEB_PORT=8080

services:
  web:
    ports:
      - "${WEB_PORT:-8080}:3000"  # Uses 8080 if WEB_PORT is unset

export WEB_PORT=8080
docker-compose up

# Valid short syntax formats
ports:
  - "3000"                    # Container port only (Docker assigns host port)
  - "3000-3005"               # Container port range
  - "8000:8000"               # HOST:CONTAINER
  - "9090-9091:8080-8081"     # Port ranges (same count required)
  - "127.0.0.1:8001:8001"     # IP:HOST:CONTAINER
  - "6060:6060/udp"           # With protocol
  - "127.0.0.1::3000"         # IP::CONTAINER (random host port)

# INVALID formats
ports:
  - "8080/udp:8080/udp"       # Wrong! Protocol goes at the end
  - "8080:8080:8080"          # Wrong! Too many colons
  - "8080:"                   # Wrong! Missing container port
  - ":8080"                   # Wrong! Missing host port

ports:
  - "12201:12201/udp"  # Correct UDP syntax

services:
  web:
    ports:
      - target: 80
        published: 8080
        protocol: tcp
        mode: host

  dns:
    ports:
      - target: 53
        published: 5353
        protocol: udp

# WRONG: Start port greater than end port
ports:
  - "8082-8080:3002-3000"

# WRONG: Mismatched range sizes
ports:
  - "8080-8082:3000-3001"  # 3 ports to 2 ports

# CORRECT: Valid range with matching sizes
ports:
  - "8080-8082:3000-3002"  # 3 ports to 3 ports

# WRONG: Spaces around equals sign
ENV PORT = 3000
EXPOSE $PORT  # Results in "Invalid containerPort: ="

# CORRECT: No spaces
ENV PORT=3000
EXPOSE $PORT

# Or use braces for clarity
ENV PORT=3000
EXPOSE ${PORT}

docker-compose build --no-cache
docker-compose up

# WRONG: Cannot use both
services:
  web:
    network_mode: host
    ports:
      - "8080:80"  # Invalid with host network

# CORRECT: Host mode - no port mapping needed
services:
  web:
    network_mode: host
    # Container uses host's network directly
    # Access via whatever port the app listens on

# OR: Use bridge mode with port mapping
services:
  web:
    ports:
      - "8080:80"  # Works with default bridge network