As discussed in #313, dokku base commands such as build, release, and
deploy require specifying both `$APP` and `$IMAGE` as arguments where
`IMAGE="app/$APP"`. Thus, the `$IMAGE` parameter is currently redudant.
This patch removes the need to specify `$IMAGE` for these base commands
and is backwards compatible. If, in the future, the base commands expect
and `$IMAGE` different than `app/$APP`, then the `$IMAGE` parameter will
be re-added for those specific commands.
HOME is not consistent if dokku is ran e.g. as root (for setup).
The DOKKU_ROOT variable will also make it possible to have the repositories stored in an arbitrary location (works-ish right now)