Fix Ubuntu Docker build script and documentation paths

[Aidenwu0209]

Summary

• reference the checked-in install-prerequisites.sh from both Ubuntu
  Dockerfiles
• update the READMEs to clone ROCm/ROCm and enter the matching Docker
  directory
• normalize FROM ... AS ... casing so Docker's build check is clean

Fixes #6542

Testing

$ docker build --check tools/rocm-build/docker/ubuntu22
Check complete, no warnings found.

$ docker build --check tools/rocm-build/docker/ubuntu24
Check complete, no warnings found.

The following static checks also completed successfully:

git diff --check
test -f tools/rocm-build/docker/ubuntu22/install-prerequisites.sh
test -f tools/rocm-build/docker/ubuntu24/install-prerequisites.sh
! rg -n 'install-prerequisities|rocm-build/build/docker' \
  tools/rocm-build/docker/ubuntu22 \
  tools/rocm-build/docker/ubuntu24

This PR runs Docker's check mode only; it does not perform the full
dependency-heavy ROCm builder image build.