boost::process::this_environment and Docker CMD in exec mode

Dear @klemens-morgenstern,

This is more a question and info for workaround than an actual bugfix request, because I can't see an error within boost.process, but perhaps you think otherwise.

We have seen a crash that is almost always reproducible, is when setting environment variable on the object returned by `boost::process::this_environment` when run inside a docker on linux.

Given such a cpp code and a docker with version 20.10.7-0ubuntu1~20.04.1 (version of boost process is old here 1.73.0 but I don't think there has been changes on this part), built with clang 11 on linux (std c++17) and code :

```cpp
class cli {
public:

cli() {
auto env = boost::this_process::environment();

env["AZURE_CORE_OUTPUT"] = "json";
env["AZURE_CORE_DISABLE_CONFIRM_PROMPT"] = "true";
env["AZURE_CORE_COLLECT_TELEMETRY"] = "false";
env["AZURE_CORE_NO_COLOR"] = "true";
}
};
```

```dockerfile
FROM ubuntu:20.04 as production

#...snip...

CMD [ "./tipibuild" ]
```

We get almost-always crashes of the following form:

```
0# tipi::util::detail::on_terminate_print_stacktrace() in ./tipibuild,
1# tipi::util::detail::on_crash(int) in ./tipibuild,
2# 0x00007F4000B1E210 in /lib/x86_64-linux-gnu/libc.so.6,
3# 0x00007F4000C5FC5E in /lib/x86_64-linux-gnu/libc.so.6,
4# boost::process::basic_environment_impl::find(std::__cxx11::basic_string, std::allocator > const&) in ./tipibuild,
5# boost::process::basic_environment_impl::operator[](std::__cxx11::basic_string, std::allocator > const&) in ./tipibuild,
6# tipi::clouds::azure::cli::cli(tipi::clouds::azure::azure_config_t) in ./tipibuild,
7# tipi::clouds::azure_windows::azure_windows(tipi::build::service::Machines&, tipi::clouds::azure::azure_config_t) in ./tipibuild,
8# void __gnu_cxx::new_allocator::construct(tipi::clouds::azure_windows*, tipi::build::service::Machines&, tipi::clouds::azure::azure_config_t const&) in ./tipibuild,
9# std::__shared_count<(__gnu_cxx::_Lock_policy)2>::__shared_count, tipi::build::service::Machines&, tipi::clouds::azure::azure_config_t const&>(tipi::clouds::azure_windows*&, std::_Sp_alloc_shared_tag >, tipi::build::service::Machines&, tipi::clouds::azure::azure_config_t const&) in ./tipibuild,
10# tipi::build::service::CloudSelector::CloudSelector(tipi::build::service::Machines&) in ./tipibuild,
11# default_tipi_app_components::default_tipi_app_components(boost::asio::io_context&) in ./tipibuild,
12# tipibuild_t::tipibuild_t(boost::asio::io_context&) in ./tipibuild,
13# main in ./tipibuild,
```

Just before crashing the process tree looks likes this :

## Workaround
if you change the dockerfile to use the shell execution mode of docker instead of the exec mode :

```dockerfile
FROM ubuntu:20.04 as production

#...snip...

CMD "./tipibuild"
```

Thanks to the small shell process in between docker and the commands it just works fine :

Do you think it can be a bug in Boost process or more like something wrong in linux containers ?