Allow to change the path between OSMOCOM_REPO_MIRROR and
OSMOCOM_REPO_VERSION. While at it, tweak related comments (comment above
the variable as usually, replace "repo" wording with "feed" for the
latest/nightly variable as we usually refer to it as feed), and mention
OSMOCOM_REPO_* in README.md.
In order to be able to use a different mirror for testsuite and systems
under test, the testsuite related Dockerfiles (osmocom-bb-host-master,
debian-stretch-titan) are not using OSMOCOM_REPO_PATH. We could add a
OSMOCOM_REPO_TESTSUITE_PATH on demand, as mentioned in the next commit.
Maintaining several versions of the same file in different folders
is a bad idea, because at some point their content gets out of sync.
This is exactly what happened to 'respawn.sh': sleep()ing was only
implemented in 'osmo-bts-master/respawn.sh', other versions of this
file would simply ignore '$SLEEP_BEFORE_RESPAWN'.
The easiest solution would be to have all common files in a single
directory, however Docker does not allow to ADD files from outside
of the build context. In other words, all files must be in the
same directory as the Dockerfile itself.
Modify 'make/Makefile' in order to copy the contents of common
directory to the current build context ('pre-build' target) and
remove it after building ('post-build' target).
Add a new centos8-obs-latest container with shared code (just like
debian-stretch-obs-latest) and adjust the package install blocks in