make it/multi-module/{a,b} actually depend on each other, serving as …

[stoerr]

I suggest turning the integration test dockerfile-maven/plugin/src/it/multi-module into an actual example of two docker images where the second one is built from the contents of the other. Currently both are built from scratch, but I suggest that multi-module/b is built from multi-module/a. Recently, I had to do something like that myself and it took me a while to figure out that you need to set dockerfile.build.pullNewerImage to true in my equivalent of multi-module/b , because otherwise it tries to pull the image a instead of using the just built local one. (Compare https://github.com/spotify/dockerfile-maven/issues/261 ).

My PR changes that multi-module/b is built from multi-module/a and sets pullNewerImage to true. I did not verify that the included docker-info of multi-module/a is actually used for something in multi-module/b - I don't know where to look and I didn't need that feature. The test in dockerfile-maven/plugin/src/it/multi-module/verify.groovy doesn't seem to verify that, anyway - that's probably room to improve.

Thanks for your very valuable tool!