util-docker: Cleanup, refactor, better document Dockerfiles (#1292)

* Removes the "docker-compose.yaml" in favor of "docker-bake.hcl". This
uses the `docker buildx` tool which has the advantage of enabling
multi-platformm builds where desired. By default all images are built
targeting `linux/arm64`, `linux/amd64` and `linux/riscv64` as targets
with the exception of the GPU images where only `linux/amd64` makes
sense.
* Remove unused/older Docker build targets (these can easily be re-added
but they were not regularly built or have any current usage).
* Update "README.md" to better describe these Dockerfiles and how they
are built.
* Simplify GCC and Clang compiler images. Each uses the Ubuntu 24.04 All
Deps image as a base then specialized the compiler on top.
* To simply things, all compiler versions are built from 24.04. This
means **narrowing the supported versions from GCC v10 to v14 and Clang
v14 to v18**.
* Fix some bugs in the "docker-bake.hcl" thus ensuring all targets may
be built from it.
* Cleanup the systemc and sst images: reducing their size and building
them off the common 24.04 ubuntu base image.

.github/workflows/ci-tests.yaml

@@ -96,7 +96,7 @@ jobs:
         # for compilation error to be exposed.
         runs-on: [self-hosted, linux, x64]
         if: github.event.pull_request.draft == false
-        container: ghcr.io/gem5/clang-version-16:latest
+        container: ghcr.io/gem5/clang-version-18:latest
         needs: [pre-commit, check-for-change-id]
         timeout-minutes: 90
         steps: