The headless client image we ship doesn't have systemd-resolved. By
default, if you try to run it without setting the `FIREZONE_DNS_CONTROL`
var, it will exit with an unhelpful error message that is difficult to
diagnose / debug for anyone unfamiliar to the inner workings of the
headless client:
```
Error: I/O error: No such file or directory (os error 2)
Caused by:
No such file or directory (os error 2)
Stack backtrace:
0: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
1: firezone_bin_shared::network_changes::imp::Worker::new_dbus::{{closure}}
2: firezone_headless_client::main::{{closure}}
3: tokio::runtime::runtime::Runtime::block_on
4: firezone_headless_client::main
5: std::sys::backtrace::__rust_begin_short_backtrace
6: std::rt::lang_start::{{closure}}
7: main
```
To fix this, we set `FIREZONE_DNS_CONTROL=etc-resolv-conf` for the user
inside our headless-client image.
---------
Co-authored-by: Thomas Eizinger <thomas@eizinger.io>
