Do not use default Docker bridge network

[melroy89]

• Use best practices for docker networks
• Do not use the default Docker bridge network
• Introduce both an internal as well as an external docker network
  • Its important to name the network uniquely, to avoid conflicts with other Docker bridge networks

Only question I have: Does the zammad-elasticsearch container need access to the Internet? If not, we can remove the external network from this container as well.

Fixes: #522