Add necessary variable defaults to Makefile
Add necessary variable defaults to Makefile

Allow 'make all' to run without having to define the
'CCP_IMAGE_PREFIX', 'CCP_IMAGE_TAG' or 'CCP_POSTGIS_IMAGE_TAG'
environment variables.

CrunchyData/crunchy-containers,CrunchyData/crunchy-containers,CrunchyData/crunchy-containers