Bring back the osmocom user that was removed earlier to fix a UID conflict with the build user.
The osmocom user is being used after all, the TTCN-3 PGW tests use it to start commands as "osmocom" user in osmo-uecups-master. This lead to e.g. PGW_Tests.TC_createSession_ping4 failing.
Rename the user instead of adding a new one, so it can write log files to the log dir as the same UID as the user that runs docker.
debian-bullseye-titan-*/Dockerfile: Replace inetutils-ping with iputils-ping
iCurrently we install inetutils-ping in some docker images and iputils-ping in others; let's make it more uniform to avoid running different implementations in different images. The iputils-ping provides further linux-specific features, such as the "-I interface" param, which is used by some images, so let's pick that one everywhere.
