ttcn3/ttcn3.sh: update DOCKER_IMG_BUILD

The testsuite is running in bullseye for some time, so build the Osmocom
programs in bullseye as well.

Depends: docker-playground I4cd05ef2c164dfb765ee365ab38b8dfd4f955ac6
Change-Id: Ie5be8dc976fb8ff37b6712f1cba434b2114c4bf8
1 file changed