Unknown MySQL server host 'db' (-2)

目的：　作成済みのrailsアプリをdocker環境で起動できるようにしたい。
現状：　mysqlエラーが発生している

Mysql2::Error::ConnectionError at /
Unknown MySQL server host 'db' (-2)

docker
1version: '3'
2services:
3    db:
4        image: mysql:5.7
5        environment:
6            MYSQL_USER: root
7            MYSQL_ROOT_PASSWORD: password
8        ports:
9            - "3306:3306"
10        volumes:
11            - ./tmp/db:/var/lib/mysql/data
12
13    web:
14        build: .
15        command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
16        volumes:
17            - .:/work
18            - gem_data:/usr/local/bundle
19        ports:
20            - 3000:3000
21        depends_on:
22            - db 
23        tty: true
24        stdin_open: true
25
26volumes:
27  gem_data:

dockerfile
1FROM ruby:2.5.1
2RUN apt-get update && apt-get install -y nodejs --no-install-recommends && rm -rf /var/lib/apt/lists/*
3RUN apt-get update && apt-get install -y mysql-client --no-install-recommends && rm -rf /var/lib/apt/lists/*
4RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
5
6RUN mkdir /work
7WORKDIR /work
8
9COPY Gemfile /work/Gemfile
10COPY Gemfile.lock /work/Gemfile.lock
11
12RUN bundle install
13COPY . /work

database.yml
1default: &default
2  adapter: mysql2
3  encoding: utf8
4  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
5  username: root
6  password: password
7  host: db
8