When using the container images, I've happened to rebuild them quite often,
especially when doing different plugin combinations. Also, when considering
using the containers in CI, it seems like a good idea to keep the base images cached,
so that every PR doesn't need to download most of the dependencies from scratch.

The proposal is to introduce Dockerfile.builder-base, that would be used to build:

• quay.io/foreman/foreman:base - the base os used in all of the other container images
• quay.io/foreman/foreman:builder-base - the image with all basic ruby and node dependencies to streamline the build process