This bumps the docker image used to build GPU applications for input to
GPUFS simulations from ROCm 5.4.2 to ROCm 6.0.2 and Ubuntu from 20.04 to
22.04. This matches the versions in gem5-resources#29 .
Several notes were added to the Dockerfile to describe where the RUN
commands come from. A README.md is also added to clarify that this is
not a disk image for GPUFS and is only used to build applications.
Change-Id: I9ada99e2ed1854cb7adb76f2a1fa662bab398f86
