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).
This is initialized to docker.io, keeping the default behaviour
if not specified. However, it allows us to specify a private
registry later on.