Fix Dockerfile files

[simonpasquier]

This change uses multistage builds to generate the container images. It also adds the .rhel7 Dockerfile for OCP builds.

This is similar to openshift/cluster-monitoring-operator#147.

cc @s-urbaniak @pgier

[pgier]

/lgtm, @simonpasquier I guess this will fix #13?
Also, after this is merged we'll have to update the Dockerfile setting here: https://github.com/openshift/ocp-build-data/blob/master/openshift-4.0/images/prometheus-operator.yml

[s-urbaniak]

/lgtm

looks great, thanks a lot @simonpasquier !

[simonpasquier]

Also, after this is merged we'll have to update the Dockerfile setting here: https://github.com/openshift/ocp-build-data/blob/master/openshift-4.0/images/prometheus-operator.yml

Yep this was my idea too. Will try ASAP.