1. docker-compose.yaml:
version: "3.8"

services:
  redis-1:
    build: ./redis-server
    volumes:
      - ./redis-server/conf/dev.conf:/usr/local/etc/redis/redis.conf

  redis-2:
    build: ./redis-server
    volumes:
      - ./redis-server/conf/dev.conf:/usr/local/etc/redis/redis.conf

  redis-3:
    build: ./redis-server
    volumes:
      - /home/h0tw4t3r/2-docker/redis-cluster/redis/conf/dev.conf:/usr/local/etc/redis/redis.conf

  sentinel-1:
    build: ./redis-sentinel
    volumes:
      - ./redis-sentinel/conf:/bitnami/redis-sentinel/conf
    ports:
      - "26379:26379"

  sentinel-2:
    build: ./redis-sentinel
    volumes:
      - ./redis-sentinel/conf:/bitnami/redis-sentinel/conf
    ports:
      - "26380:26379"

  sentinel-3:
    build: ./redis-sentinel
    volumes:
      - ./redis-sentinel/conf:/bitnami/redis-sentinel/conf
    ports:
      - "26381:26379"

networks:
  default:
    name: redis_cluster

2. Tree
.
├── dev-rm.sh
├── dev-start.sh
├── docker-compose.yaml
├── prod-rm.sh
├── prod-start.sh
├── redis-sentinel
│   ├── conf
│   │   └── dev.conf
│   └── Dockerfile
└── redis-server
    ├── conf
    │   ├── dev.conf
    │   └── prod.conf
    └── Dockerfile

4 directories, 10 files

3. Sentinel Dockerfile
# syntax=docker/dockerfile:latest

FROM bitnami/redis-sentinel

EXPOSE 6973/tcp
ENTRYPOINT redis-sentinel /usr/local/etc/sentinel/sentinel.conf

4. Result
# docker-compose run sentinel-1
6:X 09 May 2020 14:06:53.215 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
6:X 09 May 2020 14:06:53.215 # Redis version=6.0.1, bits=64, commit=00000000, modified=0, pid=6, just started
6:X 09 May 2020 14:06:53.215 # Warning: no config file specified, using the default config. In order to specify a config file use redis-sentinel /path/to/sentinel.conf
                _._
           _.-``__ ''-._
      _.-``    `.  `_.  ''-._           Redis 6.0.1 (00000000/0) 64 bit
  .-`` .-```.  ```\/    _.,_ ''-._
 (    '      ,       .-`  | `,    )     Running in sentinel mode
 |`-._`-...-` __...-.``-._|'` _.-'|     Port: 26379
 |    `-._   `._    /     _.-'    |     PID: 6
  `-._    `-._  `-./  _.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |           http://redis.io
  `-._    `-._`-.__.-'_.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |
  `-._    `-._`-.__.-'_.-'    _.-'
      `-._    `-.__.-'    _.-'
          `-._        _.-'
              `-.__.-'

6:X 09 May 2020 14:06:53.219 # Sentinel started without a config file. Exiting...