Fix switch to nonroot user in service-catalog's Dockerfile

[skinlayers]

Tested in:

• Windows 11 + WSL2 + Ubuntu 24.04 + Docker Desktop
• MacOS X 15.5 + Docker Desktop (Intel & Apple Silicon)

Building the service-catalog image fails when ran locally, due to its Dockerfile referencing a "nonroot" user account that does not exist in the base image. The PR fixes this by using the "nobody" account instead.

$ docker compose -f docker-compose.yaml up --build
[...]
dummy_service-1    | time=2025-08-04T05:23:14.233Z level=ERROR msg="gRPC request handled" service=products source=grpc grpc_service=grpc.ProductService grpc_method=updateProductStock grpc_code=InvalidArgument duration_ms=16
Error response from daemon: unable to find user nonroot: no matching entries in passwd file

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