The mcds CardDav client does a fine job of looking up NextCloud directory entries on behalf of the mutt e-mail client but was marginally awkward to build on account of autoconf's ... unfriendliness. In the end it built happily on Debian 10, 11 and 12 but I didn't get it working on Debian unstable so here's a Debian 12-based container!
Having pulled the docker image I created a directory with a suitable .muttrc and .netrc and mapped it in on the command invoking the tool in the mutt configuration:
set query_command="docker run -v ~/opt/mcds:/root -t quay.io/ajb85/mcds:deb12-i386 mcds %s"
GPG integration and support for the man page are removed to save 10-15MB on the image size. These can be brought back in by uncommenting the relevant lines in the container definition.