https://github.com/apereo/mod_auth_cas/issues/97
in some cases simply running
$ autoreconf -f -i
and nothing else .... solves the problem.
You do that in the directory /pcre2-10.30
.
What a nightmare.
(This usually did not solve the problem in 2017, but now usually does seem to solve the problem - they fixed something. Also, it seems your Dockerfile should now usually start with "FROM ibmcom/swift-ubuntu" ; previously you had to give a certain version/dev-build to make it work.)