Add a check if Docker is available on the system

[fusion44]

flutter: ProcessException: No such file or directory
flutter: Command: docker ps -a
[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: ProcessException: No such file or directory
Command: docker ps -a
#0 _ProcessImpl._start (dart:io-patch/process_patch.dart:402:33)
#1 Process.start (dart:io-patch/process_patch.dart:38:20)
#2 _runNonInteractiveProcess (dart:io-patch/process_patch.dart:579:18)
#3 Process.run (dart:io-patch/process_patch.dart:49:12)
#4 getRunningContainerNames (package:regtest_core/src/docker/docker.dart:76:30)
#5 NetworkManager._checkIfRunning (package:regtest_core/src/manager.dart:138:30)
#6 NetworkManager.init (package:regtest_core/src/manager.dart:73:11)
#7 NetworkBloc._init (package:regtest_app/blocs/network_bloc/network_bloc.dart:37:39)
#8 new NetworkBloc (package:regtest_app/blocs/network_bloc/network_bloc.dart:16:5)
#9 main (package:regtest_app/main.dart:55:16)