Move modules-update to custom_build and gate redis_repo

[dariaguy]

Note

Medium Risk
Changes release Docker build behavior (no modules-update on release images) and CI validation for redis_repo; mistakes could affect official image contents or block legitimate release runs.

Overview
Release automation now rejects run_type=release when redis_repo is not redis/redis, so forked Redis sources are only allowed for custom/unstable/nightly-style runs.

Docker image builds drop the bundled-module modules-update / .prepared shortcut from the checked-in release Alpine and Debian Dockerfiles; those paths go straight from LTO setup to Rust install and make deploy.

The Jinja templates keep modules.yaml pinning, the .prepared skip, and make modules-update only under custom_build, matching how non-release renders are produced while release images rely on the official tarball’s bundled module sources.

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