internal/config: allow setting environment using environment variable

[dalen]

This fixes #502

[gbbr]

I thought we agreed within #502 that the problem was resolved and this isn't needed.

[dalen]

Well, there is a workaround with some code changes in the application. But I still think it is good to be able to set it on the agent even in docker environments.

You said in that issue

In a next release (soon) most config options will be settable via environment variables too.

So I thought it would get fixed in the env config loading refactoring, #531 but it seems it wasn't, so therefore created this PR.

[gbbr]

Thanks for the explanation. You're right, this wasn't part of those changes. Let's add it.