Better support multi-stage builds #417
Description
E.g. in the .NET component (https://github.com/merkatorgis/docker4gis-dotnet).
Currently, the template Dockerfile start with FROM xxx/dotnet/sdk:xxx as builder, followed by the instructions needed to copy in and compile the sources, then FROM docker4gis/dotnet:xxx, followed by COPY --from=builder xxx xxx.
Would be nice, if even possible (but I doubt it...), to have the build instructions as ONBUILD in the builder image, and have the template Dockerfile just do:
FROM xxx/dotnet/sdk:xxx as builder
FROM docker4gis/dotnet:xxx
COPY --from=builder xxx xxx