File size checks fails and I haven't figured out why, but next I'd look into how `posix_fallocate` works on alpine linux.

Log:

```
$ cat /etc/alpine-release 
3.9.2

$ git log --oneline -1
e45b7f20e Merge #15618: refactor: Remove unused function

$ ./src/test/test_bitcoin -t flatfile_tests
Running 4 test cases...
Version using posix_fallocate
test/flatfile_tests.cpp(94): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 0))) == 100 has failed [0 != 100]
test/flatfile_tests.cpp(98): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 100 has failed [0 != 100]
Version using posix_fallocate
test/flatfile_tests.cpp(102): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 200 has failed [0 != 200]
Version using posix_fallocate
test/flatfile_tests.cpp(116): error: in "flatfile_tests/flatfile_flush": check fs::file_size(seq.FileName(FlatFilePos(0, 1))) == 100 has failed [0 != 100]

*** 4 failures are detected in the test module "Bitcoin Core Test Suite"
```
error = "Failed to include configuration file " + to_include;
                    return false;
                }
            }
            
            includeconf = GetArgs("-includeconf");
            {
                std::vector<std::string> includeconf_net(GetArgs(std::string("-") + chain_id + ".includeconf"));
                includeconf.insert(includeconf.end(), includeconf_net.begin(), includeconf_net.end());
                std::string chain_id_final = GetChainName();
                if (chain_id_final != chain_id) {
                    
                    includeconf_net = GetArgs(std::string("-") + chain_id_final + ".includeconf");
                    includeconf.insert(includeconf.end(), includeconf_net.begin(), includeconf_net.end());
                }
            }
            for (const std::string& to_include : includeconf) {
                fprintf(stderr, "warning: -includeconf cannot be used from included files; ignoring -includeconf=%s\n", to_include.c_str());
            }
        }
    }
    
    ClearDatadirCache();
    if (!fs::is_directory(GetDataDir(false))) {
        error = strprintf("specified data directory \"%s\" does not exist.", gArgs.GetArg("-datadir", "").c_str());
        return false;
    }
    return true;
}
std::string ArgsManager::GetChainName() const