ttcn3-testsuites: Set REGISTRY_HOST="registry.osmocom.org"

This will make the Dockerfiles from docker-playground.git
use registry.osmocom.org rather than the public docker.io for base
distribution images like debian:stretch or the like.

This will avoid any strange timeout/EOF problems while doing
docker build --pull in our test jobs.

Change-Id: If4275cf3c8a5b70a67d742950a35ae8ead59a52e
Closes: OS#4839

jobs/ttcn3-testsuites.yml

@@ -196,6 +196,7 @@
 
     builders:
       - shell: |-
+          export REGISTRY_HOST="registry.osmocom.org"
           case "{job-name}" in
               TTCN3-centos-*-latest)
                   cd "$(echo "{job-name}" | sed s/^TTCN3-centos\-/ttcn3-/ | sed s/\-latest$//)"