title: How to change the default character set of mysql using docker-compose?
body: <p>When I save the string of Chinese, mysql rise the error "Exception Value:<br>
  (1366, "Incorrect string value: '\xE5\xB0\x8F\xE6\x98\x8E' for column 'name' at row 1")",I check the character of mysql,it show this:</p>
  
  <pre><code>mysql&gt; show variables like 'character%';
  +--------------------------+----------------------------+
  | Variable_name            | Value                      |
  +--------------------------+----------------------------+
  | character_set_client     | latin1                     |
  | character_set_connection | latin1                     |
  | character_set_database   | latin1                     |
  | character_set_filesystem | binary                     |
  | character_set_results    | latin1                     |
  | character_set_server     | latin1                     |
  | character_set_system     | utf8                       |
  | character_sets_dir       | /usr/share/mysql/charsets/ |
  +--------------------------+----------------------------+
  8 rows in set (0.00 sec)
  </code></pre>
  
  <p>And my docker-compose.yml is as fellow:</p>
  
  <pre><code>web:
      image: yetongxue/docker_test:1.2
      links:
        - "db"
      ports:
        - "8100:8000"
      volumes:
        - "/Users/yetongxue/docker_v/docker_test/media:/root/media"
      restart: always
  
  db:
      image: mysql:5.7
      environment:
        MYSQL_ROOT_PASSWORD: qwerasdf
        MYSQL_DATABASE: docker_db
      restart: always
      volumes:
        - "/Users/yetongxue/docker_v/docker_test/db:/var/lib/mysql"
  </code></pre>
  
  <p>I know how to set the character of mysql with my.cnf,but how can I do this in the docker-compose.yml?
  Anybody know this? Thanks!</p>