chore: Custom GitHub runner image with Unreal docker pre-installed - #1340

This PR adds custom GitHub runner image generation workflows aimed at reducing CI times. Instead of pulling large Unreal Engine Docker images at the beginning of every job, they are pre-pulled and baked into reusable runner snapshots. As a result, build/test checks can start with the required Docker images already available locally, eliminating image download time and reducing network-related variability.

Custom Image Generation

Three dedicated, single-purpose image-builder workflows are introduced, each producing its own named snapshot:

Each workflow runs on workflow_dispatch and a weekly schedule. During execution it:

• Frees up disk space on the builder runner.
• Authenticates with GHCR.
• Pulls the required Docker images, logging available disk space after each pull.
• Verifies that all expected images were pulled successfully.
• Creates a GitHub Actions custom image snapshot from the prepared environment.

Custom image generation runner specs:

• 8 CPU cores
• 32GB RAM
• 300 GB SSD
• x64 arch

The set of Unreal Engine versions included in the pre-built images is constrained by the available disk space on the image-generation runners. Priority is given to the three latest engine versions, while UE 4.27 is retained to maintain backward compatibility.

Runners that consume custom images (belong to Large Runners for GDX group):

• windows-latest-unreal
• ubuntu-latest-unreal
• android-latest-unreal

Build Time Optimization

As Unreal sample project packaging now becomes the primary contributor to CI execution time, the following optimizations were introduced to improve its performance:

• Forced Unreal Build Tool to use all available logical CPU cores for compilation (-UBTArgs="-AllCores").
• Removed recursive submodule checkout from build jobs.
• Enable unity build when running checks against PR's branch.

Benchmarks

Per-platform build time comparison before and after integrating custom GitHub runner images and implementing the CI optimizations described above.

Windows

Linux

Android

Estimated Cost

Based on the average build times above, the per-platform test matrix consumes approximately 290 minutes of runner time and costs an estimated $8.16/run ($2.35 Linux + $3.37 Windows + $2.44 Android) on 8-core runners (rates: $0.022/min Linux & Android, $0.042/min Windows - confirm against Actions runner pricing).

In order to further reduce CI execution time and compute costs, the following optimizations were applied:

• Enabled cancel-in-progress for pull request workflows, ensuring that in-flight jobs are automatically canceled when newer commits are pushed to the same PR. Workflows running on main and release branches continue to run to completion regardless of subsequent commits.
• Split the test job matrix so that pull requests run a reduced set of engine versions (UE 4.27 plus the three latest Unreal Engine releases) while full coverage across all supported versions is retained for main and release branch workflows.

With these changes in place, the pull-request test matrix (the most common execution path) drops to approximately 190 minutes of runner time, an estimated $5.44/run ($1.26 Linux + $2.68 Windows + $1.50 Android).

Other Considerations

• Currently, image regeneration is scheduled to run weekly per GitHub's recommendation. Storage for custom images is billed through GitHub Actions storage. By default, unused images stored for 30 days (see retention policy).
• sentry-unreal is a public repository with access to the image generation runners. If this ever become a security concern these workflows can be moved to a private one.

#skip-changelog