Thank you so much for your help! This enabled me to investigate further, although I did not find the cause or a solution… It it really strange:
- Yes, the IP 149.172.19.157 was my own external IP at home I got from my internet provider
- Using busybox in the same namespace (smart-home) nslookup fetches the correct addresses for dl.bintray.com. I played around with dnsPolicy and dnsConfig for busybox and it always worked correctly regardless if the cluster DNS or my DNS at home (192.168.17.104) were used
- I connected to the openHAB container with /bin/bash and examined /etc/resolv.conf and it contained the correct settings (cluster DNS first then my home DNS, correct search domains). arping in this container returned the wrong (my own external) IP. I tried to install nslookup using apt-get in the openHAB container but all external names such as prod.debian.map.fastly.net were resolved to my own external IP.
I don’t know what’s wrong here. When I connect to my Node RED container with /bin/bash it has the exact same configuration in /etc/resolv.conf as openHAB. It has nslookup installed and resolves everything correctly: bintray, debian mirror, local names, cluster names. 
The openHAB container only resolves cluster names correctly, local names in my home network and external names are all resolved to my external IP at home. 