Revert docker installation and add golang and RBE_CONFIG_CONTAINER in… (#2748)

… ubuntu2404 image

With dual execution mode support (--exec_mode=host) added to
rbe_config.bzl (#2747), Docker-in-Docker is no longer needed in CI
containers for dynamic RBE toolchain generation.

This commit updates buildkite/docker/ubuntu2404/Dockerfile:
- Reverts #2642 by removing `docker.io` from apt-get install.
- Adds `golang` so that rbe_configs_gen can be compiled natively from
source in host mode.
- Sets `ENV
RBE_CONFIG_CONTAINER="gcr.io/bazel-public/ubuntu2404:latest"` so that
rbe_config.bzl automatically activates host mode when running inside
this container.