Add optional `--redis-tag` flag, fixes #1

[stasadev]

The Issue

• #1
• https://github.com/ddev/ddev/issues/4430

How This PR Solves The Issue

Adds optional --redis-tag value flag, that creates .ddev/.env.redis file with REDIS_TAG="value".

Manual Testing Instructions

Install add-on using DDEV binary from:

• https://github.com/ddev/ddev/pull/6406

ddev add-on get https://github.com/ddev/ddev-redis/tarball/20240717_stasadev_optional_version
ddev dotenv set .ddev/.env.redis --redis-tag=7
ddev restart

Automated Testing Overview

Related Issue Link(s)

Release/Deployment Notes

[rfay]

Please take a look and make ready for review when ready, thanks.

[stasadev]

Sure, but first I need to finish the parent task and it should probably be ready next week.

[rfay]

Works great for me, can be pulled when we're ready.