I read many many posts on discourse.snowplow.io and couldn’t believe that there was an issue or bug in Snowplow mini or Iglu itself, so I started looking further afield - specifically into the Java error. That led me to this StackOverflow post:
It turns out that the issue is with Java not liking underscore characters _ in hostnames, and I was using iglu_server as my docker image name. Updating this to be iglu-server (- instead of _) solved my issue!
Sharing here so others may find sanity.