Both prepare-time probing and launch-time race-retry generated the
same `<base>, <base>-2, ..., <base>-N` sequence with their own copies
of the suffix arithmetic and the 99-cap. Extract the candidate stream
into docker/util.container_name_candidates and have both call sites
walk it; each keeps its own predicate (probe vs. retry).
Also bumps the cap into a named constant (MAX_CONTAINER_SUFFIX) so
the two error messages can't drift.
didericis
42c2e8108e