RHEL8.2 dockerfiles: create intermediary build image

This commit creates an intermediary build image for all eNB, gNB, UE,
nrUE, RU targets (ran-build) after the image containing all RAN
compilation dependencies (ran-base). Then, the corresponding target
container images are just copied from the ran-build.

The only exception is phySim, which is still compiled independently, as
the build_oai would otherwise put all compiled files into
phy_simulators/build instead of ran_build/build. We do this at this
state to avoid any inconsistencies.