chore(frontend): remove stale Dockerfile and start-nginx script (#12795)

These files were superseded by docker/frontend/ in Dec 2024 but left in
place. They still use the pre-openshift sed -i entrypoint that breaks
under readOnlyRootFilesystem, so any local build from this Dockerfile
produces a broken image. Nothing in CI, Makefile, or build tooling
references them.

Author: ogabrielluiz