Changes

sync debian-buster-build with debian-stretch-build

Change-Id: I6f871613fc4e8dd069cbe8c4a0ef3d1725be8c7a

Add debian-bullseye-build container

Change-Id: Ie2c75f30a5fc46979812ddfbea349e3d2fa91a7d

debian-{buster,stretch}-*: Explicitly specify DISTRO variables

It seems a manual 'make' in the respective directories no longer
works, as always the default distro is used as upstream reference.

Let's work around this by adding DISTRO variable assignments to the
respective makefiles.

Change-Id: I8769d504ca7afde07d4a0ad1f03aaaec892bf576

bsc-test: Reduce log verbosity of osmo-stp

I don't really see the point of crating 19 MBytes of logs for every
ttcn3-bsc-* test run that consists mainly of debug messages about
sending a message from left to right. osmo-stp is not the IUT here,
but merely a part of the test fixture.  Let's reduce log verbosity.

Change-Id: If1d22814d89c4e52b3b7804110256d896b7cc99f