Pre-build system for docker images

[chayim]

Currently on a per-platform basis we use a base docker image. This image covers creating a pre-build system for per-product dockers, extending the approach started in rsbuilder. Tags will be pushed to dockerhub using the same versioning strategy defined in #22, but in the redisfab organization. The key difference will be the $PRODUCT names, as the base dockers will be $PRODUCTbuild (i.e redisgearsbuild). As noted in #22 these will be per branch/version of each product.

These images should have their own automated build cycle where:

1. Automatically pulling from the branch, where a change in the system-setup for the associated product, will trigger a build of the docker itself. This is likely a github action given it's support for per-file triggers.

2. A weekly run will occur, purely for re-building the base image of the underlying product.

3. A forced build will be available, due to a product specific series of changes that may necessitate a manual build. This likely ties in to 1 above.