Add license for bibliographic data (third attempt)

[Flowdalic]

We were asked [1] to state that the XEP bibliographic data is openly available and free of charge if we want it to be consumed by third parties.

This weas previously merged as 747738804a2d ("Add license for bibliographic data"), but unfortunately later reverted in 59b2a5ca15c9 ("Revert "Add license for bibliographic data"", https://github.com/xsf/xeps/pull/1221). However, the fix for the broken docker build is trivial: simply also copy refs-LICENSE into the container. This commit does that.

Fixes #1219.

1: https://github.com/ietf-tools/bibxml-service/issues/302#issuecomment-1274686153