Add a flag to skip rebuilding the whole chunk Dockerfile for changes in `tests/`

[axonasif]

While trying to contribute to https://github.com/gitpod-io/workspace-images I find myself quite slowed down each time I try to make some changes to its tests/ and then running dazzle. I'm not sure why rebuilding the Dockerfiles is necessary for changes made in tests/.

cc @princerachit