container: default FROM_IMAGE to Rocky Linux 10

Since tentacle, the preferred base image for ceph containers has been
Rocky Linux 10, and the CI tag-naming logic in build.sh already assumes
rockylinux-10 is the default fromtag for every branch except reef and
squid.  The actual build default was never flipped, though: anything
that ran build.sh without FROM_IMAGE set (e.g. the release container
job in ceph-build) still got a CentOS Stream 9 base.

Flip the Containerfile ARG and the build.sh fallback to
docker.io/rockylinux/rockylinux:10 so umbrella and later build from
Rocky 10 by default.  Builds that want a different base can still pass
FROM_IMAGE explicitly, as the CI pipeline does.

Signed-off-by: David Galloway <david.galloway@ibm.com>