Split build-runtime into build-{base,runtime,sdk}

Split the creation of base, platform and sdk runtime as process fails to
access to network on 2nd call to debos, with the following errors:
  Error: Os { code: 98, kind: AddrInUse, message: "Address already in use" }
  Core dump limits :
  	soft - NONE
  	hard - NONE
  Checking that ptrace can change system call numbers...OK
  Checking syscall emulation patch for ptrace...OK
  Checking advanced syscall emulation patch for ptrace...OK
  Checking environment variables for a tempdir.../scratch
  Checking if /scratch is on tmpfs...OK
  Checking PROT_EXEC mmap in /scratch...OK
  Adding 3395584 bytes to physical memory to account for exec-shield gap
  Event loop failed: Connection timed out
  …
  2020/09/09 07:04:39 apt-install | E: Failed to fetch https://repositories.apertis.org/apertis/pool/hmi/m/mildenhall/mildenhall_0.2020.1bv2021dev1b1_all.deb

  Temporary failure resolving 'repositories.apertis.org'
  2020/09/09 07:04:39 apt-install | E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
  2020/09/09 07:04:39 apt-install | Container root failed with error code 100.
  2020/09/09 07:04:39 apt-install |
  2020/09/09 07:04:39 apt-install | ERROR:flatdeb.apt-install:Command '['systemd-nspawn', '--directory=/scratch/root', '--as-pid2', '--tmpfs=/run/lock', '--register=no', 'env', 'DEBIAN_FRONTEND=noninteractive', 'apt-get', '-y', '--no-install-recommends', 'install', 'libc6:amd64', 'libstdc++6:amd64', 'libsystemd0', 'mildenhall']' returned non-zero exit status 100.
  2020/09/09 07:04:39 Action `run` failed at stage Run, error: exit status 1

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>