fix(ci): use shared docker builder again

I noticed none of our eic_container builds ever hit the cache anymore, not even for the base layers. That means there is no layer reuse, ever, and all these pipeline containers are indeed independent and at full size. I think this was introduced with f14905f6 since the builder holds the layer cache state. By individualizing the builder for each pipeline and removing the builder again, the layer cache is removed too.

This PR reverts to using a single shared docker builder.