registry v3 documentation does not mention MasterName option for Redis that is required for Sentinel to work

[afunix]

Description

UniversalOptions of go-redis mention MasterName option ( https://pkg.go.dev/github.com/redis/go-redis/v9#UniversalOptions ) which is used to enable Redis Sentinel.

Redis configuration options section ( https://github.com/distribution/distribution/blob/main/docs/content/about/configuration.md#redis ) does not mention this option, so there is no way for a regular user to find out how to enable Sentinel.

Reproduce

1. start registry version 3.0.0 with REGISTRY_REDIS_OPTIONS_ADDRS configured to multiple hostnames of Redis Sentinel cluster
2. Registry reports only using redis as cache, nothing appears in Redis database:

time="2025-05-19T22:11:56.305491792Z" level=info msg="using redis blob descriptor cache" environment=development go.version=go1.23.7 instance.id=5555f949-d40b-44ad-b828-35134aadd751 service=registry version=3.0.0

Expected behavior

Registry is able to discover Redis Sentinel nodes, eg:

redis: 2025/05/19 22:12:04 sentinel.go:762: sentinel: discovered new sentinel="XXXX:26379" for master="YYYY"
redis: 2025/05/19 22:12:04 sentinel.go:762: sentinel: discovered new sentinel="XXXX:26379" for master="YYYY"
redis: 2025/05/19 22:12:04 sentinel.go:762: sentinel: discovered new sentinel="XXXX:26379" for master="YYYY"
redis: 2025/05/19 22:12:04 sentinel.go:762: sentinel: discovered new sentinel="XXXX:26379" for master="YYYY"
redis: 2025/05/19 22:12:04 sentinel.go:762: sentinel: discovered new sentinel="XXXX:26379" for master="YYYY"

Keys starts to appear in Redis DB.

registry version

3.0.0

Additional Info

setting REGISTRY_REDIS_OPTIONS_MASTERNAME to a correct name enables Sentinel in go-redis

[milosgajdos]

Feel free to open a PR for docs update.