[CX] Avoid rebuilding docker image if there's an existing one

We need to make a change in the docker image build lifecycle in our docker test framework.

Docker image build is expensive and we want to avoid building it unless absolutely necessary.

So the proposal is to build the image only if an existing one cannot be found. If an existing one (latest one) is found then it'll be used by the test framework. We'll also introduce a new build property 'cbas-docker-test.force' which if set to 'true' will force the image to be rebuild (the default is 'false'). 

Therefore if the image rebuild is necessary then the user can either set that property to 'true' when running with 'cbas-docker-test' profile or alternatively can just remove all existing images with the following command:

docker image prune --filter "label=cbas-docker-test"