🐳 (docker): add redis-sentinel

[douglasduteil]

---

Problem

Local dev has only a plain redis-pwd (port 6379). Production runs a
1+1+3 sentinel cluster for HA, but there is no way to exercise the ioredis
sentinel code path locally.

Proposal

dks switch redis-sentinel — local replica of the production HA topology
so the ioredis sentinel code path can be exercised
without deploying to k8s.

Redis Sentinel is Redis's built-in HA mechanism.
Sentinels are independent processes that monitor a master/replica pair
and coordinate a failover — promoting the replica to master — when the
master becomes unreachable. A quorum (the minimum number of sentinels
that must agree the master is down before acting) prevents false
failovers from transient network issues. At least 3 sentinels are
required so quorum (N/2+1 = 2) can still be reached if one sentinel is
lost. See understanding sentinels.

• redis-sentinel-master / redis-sentinel-replica —
  replication pair; the replica is the standby that
  sentinels can promote on failover.
• redis-sentinel-1/2/3 — three sentinel processes monitoring mymaster,
  quorum 2; mirrors the 1+1+3 topology running in production.
• redis-sentinel stack — aggregator that starts the full cluster in one
  command; waits for all nodes to be healthy before the stack is
  considered up.
• core-sentinel — core variant whose env points at the sentinel cluster
  (Redis_SENTINELS, Redis_NAME) instead of the standalone redis-pwd;
  keeps hostname: core so nginx routes transparently without a new vhost.

[jonathanperret]

J'ai la même remarque et proposition que là : #1263 (comment)