Add openssh-client for remote repositories

[thannaske]

Adding openssh-client for remote repositories (solving issue #1)

[DArtagan]

Hi @thannaske, I appreciate your contribution.

Sure enough, it seems we need OpenSSH-client if we want borg SSH remotes, thanks. I want to this image to enable all of borg backup's functions and stay as minimal as possible. Is including bash also necessary?

P.S. Log of previous implementation which demonstrates that installing SSH would be necessary for connecting to SSH based remotes:

$ borg init root@172.18.0.2:/media
Local Exception.
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/borg/archiver.py", line 2052, in main
    exit_code = archiver.run(args)
  File "/usr/lib/python3/dist-packages/borg/archiver.py", line 1997, in run
    return func(args)
  File "/usr/lib/python3/dist-packages/borg/archiver.py", line 77, in wrapper
    lock_wait=self.lock_wait, lock=lock, append_only=append_only, args=args)
  File "/usr/lib/python3/dist-packages/borg/remote.py", line 188, in __init__
    self.p = Popen(borg_cmd, bufsize=0, stdin=PIPE, stdout=PIPE, stderr=PIPE, env=env)
  File "/usr/lib/python3.5/subprocess.py", line 676, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1282, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'ssh'