Custom database, custom ports and custom docker volumes path

[Bleala]

Hello there!

I really like your version of mailpiler in a docker container, the installation script is REALLY useful! 👍

I added a few things, because I needed some customization and I think this is also very useful for other people.

1. Custom MySQL Connections: so that I (and others) can use their existing database servers (if they want to).
   
   I also added a small piler.sql file, so that everyone can import the database necessary tables/structure.
   
   There is also an instruction in the README.md.
   
   And I added two new docker-compose files (piler-default-no-mysql.yml, piler-ssl-no-mysql.yml), so no mysql container starts, if you use your own database.

2. Custom Port Mapping: I wanted to map the SMTP, HTTP and HTTPS ports differently, because I run a revery proxy on my server, so you can set these ports during the installation script.
   
   Also if the SMTP port is not 25, postfix won't be uninstalled on the host (if someone would need it).

3. Custom Docker Volumes Path: If needed you can change the path of the docker volumes folder.
   
   This is very useful for me, because my docker volumes folder has a different path.

I tested everything and there were no problems (default values are the same as before), so no breaking changes.

Hope you like that.

Greetings

[Bleala]

Hello!

I updated the PR with your current changes from the main branch, if you wanna merge it.

Greetings