Contribute changes upstream and remove our fork of atmoz/sftp

[paulirwin]

In the spirit of being good OSS citizens, I'd like to offer to contribute our changes to atmoz/sftp upstream. These changes include:

• The ability to use a mounted volume directory for SSH host keys (and copy generated ones to it if empty)
  • Note: we should probably be more Linux-proper and mount this to somewhere in /mnt instead of /etc/ssh_volume. This should also probably be an env var to allow for overriding the mount location.
• The ability to chown the user directory if mounted (this should also be changed to use an env var specifying whether to do this or not)
• Arm64 build of the Docker image

If all of these changes are accepted upstream and a new release is made available with them, we can then remove our fork and just use it.