Generate the .repo file on the fly instead of downloading it from the OBS repository. Ensure the gpgkey gets downloaded via HTTPS. I'm about to use centos8-obs-latest with an alternative repository, which does not have the .repo file. Adjust other containers for consistency.
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.
Make it possible to set a different mirror for debian-stretch-titan than for all other containers. 2021q1 doesn't have the eclipse-titan package and it doesn't make sense to build it there.
I've thought about adding OSMOCOM_REPO_TESTSUITE_PATH and OSMOCOM_REPO_TESTSUITE_VERSION too, but we don't have any use for these right now. Let's add them later if we should need them.
