Support different "flavours" of base components #418
Description
Have dg build of base components read an optional list of build-arg values, so that it builds and pushes different versions of the base component image.
E.g. docker4gis/dotnet:v0.0.12-6.0.16 and docker4gis/dotnet:v0.0.12-7.0.203,
and docker4gis/postgis:v0.0.9-15-3.3 and docker4gis/postgis:v0.0.9-14-2.5.