docker cache - #1014

Motivation:

Every docker compose build in ci-build.yml, ci-pr.yml, ci-deploy.yml and
ci-release.yml rebuilds the CentOS6/CentOS7-aarch64/Debian7/AL2023 images
from scratch on each run since GitHub-hosted runners are ephemeral and
have no persistent layer cache. ci-build.yml/ci-pr.yml previously worked
around this with a type=gha buildx cache scoped per matrix leg;
ci-deploy.yml and ci-release.yml had no docker layer caching at all.

Modifications:

Switch to buildx with a registry cache backend (ghcr.io) so the
expensive image layers (devtoolset, OpenSSL built from source,
cross-compile toolchains) are reused across runs and across workflows.
Cache images are pushed to ghcr.io/netty/netty-tcnative-build-cache,
distinct from netty/netty's own ghcr.io/netty/netty-build-cache, so the
two repositories' caches can never collide or overwrite each other.

Result:

Speed up CI builds.