DEBUG build images should be tagged differently from release build

Currently the container build gives debug and release images the same tag names. This is fine for development, since you'll generally be working on one build at a time, but is confusing if you need to integrate this into a more general build process, since debug and release images are really separate artifacts.

One example I ran into is a case when I ran ./buildall.sh ... -release_and_debug, then wanted to build docker images for the release artifacts. This is not currently possible in a sane way.

I think we should tag images with a -debug suffix if they were generated from debug artifacts. start-impala-cluster would need to be smart about picking the image to use. We may want to have different build targets to build the different flavours of image, too.

Note that there are many flavours of debug images, e.g. ASAN, but I think simply separating release and debug would avoid a lot of confusion.