tools/toolchain: update maintainer instructions
tools/toolchain: update maintainer instructions

The instructions are updated for multiarch images (images that
can be used on x86 and ARM machines).

Additionally,
 - docker is replaced with podman, since that is now used by
   developers. Docker is still supported for developers, but
   the image creation instructions are only tested with podman.
 - added instructions about updating submodules
 - `--format docker` is removed. It is not necessary with
   more recent versions of docker.

Closes #7521

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla