Update bitmap index to improve mirror performance

[kumekay]

This PR improves performance of our Git mirror by adding maintenance steps that optimize packfiles and speed up fetch/clone operations:

git repack -Ad --write-bitmap-index --pack-kept-objects
Consolidates objects into a single optimized packfile and generates a bitmap index.
→ Reduces disk usage, lowers CPU/memory load on git-upload-pack, and makes clones/fetches faster.

git multi-pack-index write --bitmap
Ensures a multi-pack-index with bitmaps is always available, even between full repacks.
→ Keeps incremental fetches efficient without requiring constant heavy repacks.

Together, these changes ensure our mirror serves shallow clones and high-volume fetches more efficiently, with significantly less resource usage under load.

[Copilot]

Pull Request Overview

This PR enhances Git mirror performance by implementing bitmap index generation and multi-pack-index management to optimize fetch/clone operations. The changes introduce automated maintenance routines that reduce resource usage and improve mirror responsiveness under high load.

Key changes:

• Added periodic bitmap index regeneration with configurable intervals
• Implemented multi-pack-index updates for incremental performance gains
• Updated configuration to support bitmap maintenance scheduling

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
mirror.go	Adds bitmap and multi-pack-index refresh functions with Git optimization commands
main.go	Implements periodic bitmap regeneration goroutine with configurable intervals
example-config.toml	Adds BitmapInterval configuration option with 10-hour default
config.go	Updates configuration parsing to handle bitmap intervals and modernizes file reading
Dockerfile	Updates Go version from 1.23 to 1.25

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[tomassebestik]

LGTM!

@kumekay When this is merged, pls remove the reboot timer from the Docker cache machine (link with details in MatterMost DM).