cmd/containerbuild: use TrimSuffix instead of TrimRight (#157)
Using TrimRight will remove all characters from `*dockerRepo` from right
to left that match a character contained on `"/"+filepath.Base(*dockerRepo)`
(the cutset) until it doesn't matches anymore.
So for example, if `dockerRepo` is `example.com/fijxu/anubis`, and
`"/"+filepath.Base(*dockerRepo)` is `/anubis`, it will remove
`u/anubis` and not just `/anubis` from `dockerRepo` because `u` is a character inside the
cutoff.