Mirror host-side user home directory rather than hard-coding /home

[JonathonReinhart]

Currently, when scubainit creates the users's $HOME directory (in the container):

scuba/scubainit/scubainit.c

Lines 650 to 653 in c6f3e62

	/* Create user home directory */
	home = path_join(USER_HOME, m_user);
	if (make_homedir(home, m_uid, m_gid) != 0)
	goto fail;

It uses a hard-coded /home:

scuba/scubainit/scubainit.c

Line 30 in c6f3e62

#define USER_HOME "/home"

This works, but creates a dissonance if the users's real (host) homedir is set to something other than /home. This is especially apparent when using $HOME in volumes:

volumes:
  $HOME/.gitconfig:                    # container path
    hostpath: $HOME/.gitconfig         # host path
    options: ro

Since $HOME is expanded in the host environment, it is wrong inside the container.

Now, similar to #213, we could add a SCUBA_USER_HOME variable which expands to the same thing used by scubainit. But I think it's easier to reason about if $HOME follows the same pattern as $PWD and the host and container paths match (even if $HOME isn't mounted automatically.)