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.

docker/Dockerfile.phySim.rhel8.2

@@ -25,11 +25,7 @@
 #
 #---------------------------------------------------------------------
 
-FROM ran-build:develop AS phy-sim-build
-
-RUN rm -Rf /oai-ran
-WORKDIR /oai-ran
-COPY . .
+FROM ran-base:latest AS phy-sim-build
 
 #run build_oai to build the target image
 RUN /bin/sh oaienv && \