feat: replace --redis-tag with --redis-docker-image

[stasadev]

The Issue

--redis-tag (REDIS_TAG) is not flexible enough, people may want to use something else instead of redis.

How This PR Solves The Issue

Adds --redis-docker-image flag for REDIS_DOCKER_IMAGE.

Manual Testing Instructions

ddev dotenv set .ddev/.env.redis --redis-docker-image=redis:6
ddev add-on get https://github.com/ddev/ddev-redis/tarball/20250425_stasadev_replace_redis_tag_flag
ddev restart

# should show 6
ddev redis INFO

Automated Testing Overview

Release/Deployment Notes