Error connecting Mysql from Go REST API with Docker Compose

I'm very new to Docker, and I'm trying to dockerize a Go REST API and MySQL database to communicate with each other using Docker Compose. I am getting the error [main] Error 1049: Unknown database 'puapp'


Docker compose:

version: '3'
services:
  db:
    build: ./mysql/ 
    restart: always
    environment:
      - MYSQL_ROOT_PASSWORD=root
    volumes:
      - db_volume:/var/lib/mysql

  api-service:
    restart: always
    build: ./
    ports:
      - "8080:80"
    environment:
      - DB_USER=root
      - DB_PASS=root
      - DB_ADDRESS=db:3306
      - DB_PROTOCOL=tcp
      - DB_NAME=puapp
    depends_on:
      - db
    links:
      - db

volumes:
  db_volume:

Dockerfile for go service:

# syntax=docker/dockerfile:1

# Build stage
FROM golang:1.16-alpine AS builder
WORKDIR /app
COPY . .
RUN go mod download
WORKDIR /app/src/main
RUN go build -o restserv


# Run stage
FROM alpine:3.13
WORKDIR /app
COPY --from=builder /app/src/main/restserv .
EXPOSE 8080

CMD "./restserv"

Dockerfile for MySQL:

FROM mysql:latest
ADD dump.sql /docker-entrypoint-initdb.d

Full code - https://github.com/bens-schreiber/restservproj
Let me know if I need to add anything

Containers will be having their own ip addresses, so API container won't be able to access mysql container over 127.0.0.1. As mentioned in the comments, you want to utilize container's names to addresses from container from another. See this page for details.