Add support for corporate firewalls

**Is your feature request related to a problem? Please describe.**
Starting Toolbox from a corporate network that blocks internet access for any path that does not use corporate HTTP Proxies (such as Squid or any other type of HTTP proxy that inspects packets) is not trivially possible.

**Describe the solution you'd like**
I would love if Toolbox had a way to configure a HTTP proxy globally or per user which the `init` script would then use to configure that proxy for the package managers inside of the containers. For example in Ubuntu/Debian, adding a file `/etc/apt/apt.conf.d/99-toolbxproxy` containing:
```
Acquire::http::Proxy "http://username:password@yourproxyaddress:proxyport";
Acquire::https::Proxy "http://username:password@yourproxyaddress:proxyport";
```

**Describe alternatives you've considered**
For some container OS's it might be enough to have `HTTPS_PROXY` `https_proxy` `HTTP_PROXY` `http_proxy` `NO_PROXY` and `no_proxy` environment variables set.

I've used the Github search function to check for similar issues, but I didn't find any.

Start by locating the init script and the existing setup for package managers inside containers. Review the Ubuntu/Debian apt proxy example and the listed HTTP_PROXY, HTTPS_PROXY, and NO_PROXY variables, then define how global or per-user settings should be applied across supported container operating systems. Done means corporate-proxy configuration works for package-manager access without manual container changes.

Written by the indexing model from the issue text.