Persist cookies to reduce wp-admin logouts

[joemaller]

Experiment with adding these environment variables to the wordpress docker-compose service:

    environment:
      - WORDPRESS_AUTH_KEY=testing to see if we can persist admin logins
      - WORDPRESS_SECURE_AUTH_KEY=testing to see if we can persist admin logins
      - WORDPRESS_LOGGED_IN_KEY=testing to see if we can persist admin logins
      - WORDPRESS_NONCE_KEY=testing to see if we can persist admin logins
      - WORDPRESS_AUTH_SALT=testing to see if we can persist admin logins
      - WORDPRESS_SECURE_AUTH_SALT=testing to see if we can persist admin logins
      - WORDPRESS_LOGGED_IN_SALT=testing to see if we can persist admin logins
      - WORDPRESS_NONCE_SALT=testing to see if we can persist admin logins