dbuild: just die when no image-id is provided
dbuild: just die when no image-id is provided

Suggested-by: Pekka Enberg <penberg@scylladb.com>
> This will print all the available Docker images,
> many (most?) of them completely unrelated.
> Why not just print an error saying that no image was specified,
> and then perhaps print usage.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20191223085219.1253342-3-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla