Multiple new features supported for the VPN connection

[bvanelli]

As discussed in #19, I implemented a couple of new functions to integrate the docker image, namely:

Features:

• Add support for custom config files for the openconnect using profile.config
• Add support for custom environment files for all connections using profile.env
• Add support for custom file and folder mounts inside the container using profile.mounts
• Use a ssh proxy instead of the python implementation for faster proxy speed
• Use squid as an http proxy for an additional HTTP proxy (as not every service supports socks5)

Fixes:

• Bump alpine version to 3.18.6

The only thing I was not 100% happy with was the fact that the startup of squid cannot wait for the start of the VPN, so I had to add a delay of 15 seconds, as I could not find a better way to do it. If this is not done, squid starts before the VPN connection and ends up not working properly.

Maybe give some thoughs regarding the changes.

Closes https://github.com/ethack/docker-vpn/issues/19