Move common code from osmo-*-master to debian-stretch-build and
centos8-build:
* add the OBS repository key (debian only)
* add the OBS repository with "nightly" feed (debian only, already done
for centos8)
* install telnet
* invalidate the cache once the repository is updated
While at it, remove deprecated MAINTAINER lines.
This fixes the need to read both the repository index of debian-stretch
and centos8 and having to invalidate the cache if either of them changes
(instead of reading just the relevant index and invalidating the cache
only if the relevant one changes).
A similar change was done with osmo-*-latest in
I443d52eba416ea298cc5421e280ad2f4ca59283b.
Related: OS#5049
Related: https://docs.docker.com/engine/reference/builder/#maintainer-deprecated
Change-Id: Id69f3eaf37f823bac0947366f27a25edd277a4cb
This allows easily changing to another osmocom repo base URL if the
default one is down.
Related: OS#4862
Change-Id: I8010b08f3dabacfb3c13a44eece6c7a490e0742e