Introduce multi-stage dockerfile

Docker image is a bit heavy (1.95GB) comparing to base rockylinux:9 image (176MB). We may benefit from multi-stage build, where srsLTE and UERANSIM repos are compiled in separate stage.

• compile srsLTE and UERANSIM in separate build stage - https://docs.docker.com/build/guide/multi-stage/
  • consider static linking if it is easily achievable with cmake/configure flags
  • install binaries/libs to /usr/local/ directory (or maybe into /opt/? you decide)
  • parallelize builds - make -j
• merge dnf operations into single RUN command; remove dnf cache afterwards
• don't install unnecessary *-devel packages in final image
• use .dockerignore to aggregate multiple COPY commands

Just by getting rid of cpp build artifacts and repos we should be able to reduce image size by over 600MB