hey @mpiggott
finally fixed it, thanks to your pointers.
nexus3 v3.58.1 and earlier needed:
proxy_pass http://localhost:58100/repository/my-docker/$request_url;
but for nexus3 v3.59.0 and later (ie v3.70.1) this seems to work:
proxy_pass http://localhost:58100/$request_url;
Many thanks!