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

[tustanivsky]

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:

Workflow	Snapshot	Builder Runner	UE versions
custom-image-windows.yml	unreal-image-windows	unreal-image-creating-runner-windows	4.27, 5.5-5.8
custom-image-linux.yml	unreal-image-linux	unreal-image-creating-runner-linux	4.27, 5.2-5.8
custom-image-android.yml	unreal-image-android	unreal-image-creating-runner-linux	5.4-5.8

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

Engine	Before Avg	After Avg	Improvement
4.27	32m 15s	14m 15s	✅ 56% faster (2.26×)
5.5	58m 14s	16m 14s	✅ 72% faster (3.59×)
5.6	51m 23s	16m 51s	✅ 67% faster (3.05×)
5.7	72m 41s	15m 33s	✅ 79% faster (4.67×)
5.8	63m 00s	17m 24s	✅ 72% faster (3.62×)

Linux

Engine	Before Avg	After Avg	Improvement
4.27	18m 05s	11m 43s	✅ 35% faster (1.54×)
5.2	17m 44s	10m 09s	✅ 43% faster (1.75×)
5.3	19m 03s	12m 29s	✅ 34% faster (1.52×)
5.4	22m 46s	14m 16s	✅ 37% faster (1.60×)
5.5	24m 01s	15m 29s	✅ 36% faster (1.55×)
5.6	24m 00s	14m 29s	✅ 40% faster (1.66×)
5.7	23m 10s	14m 29s	✅ 37% faster (1.60×)
5.8	26m 00s	13m 48s	✅ 47% faster (1.88×)

Android

Engine	Before Avg	After Avg	Improvement
5.4	33m 08s	20m 00s	✅ 40% faster (1.66×)
5.5	30m 32s	22m 32s	✅ 26% faster (1.35×)
5.6	31m 55s	22m 51s	✅ 28% faster (1.40×)
5.7	31m 32s	22m 01s	✅ 30% faster (1.43×)
5.8	33m 46s	23m 26s	✅ 31% faster (1.44×)

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).

#skip-changelog

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Entries from this PR are highlighted with a left border (blockquote style).

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Bug Fixes 🐛

• Prevent potential script injection in crash-reporter-build workflow by fix-it-felix-sentry in #1324

Internal Changes 🔧

Deps

• Update Native SDK to v0.13.6 by github-actions in #1339
• Update Android Gradle Plugin to v6.4.0 by github-actions in #1338
• Update Cocoa SDK to v9.10.0 by github-actions in #1335
• Update Java SDK to v8.38.0 by github-actions in #1336
• Update Native SDK to v0.13.5 by github-actions in #1334
• Update Cocoa SDK to v9.9.0 by github-actions in #1332
• Update CLI to v3.3.5 by github-actions in #1323

Other

• Custom GitHub runner image with Unreal docker pre-installed by tustanivsky in #1340

• Update validate-pr workflow by stephanie-anderson in #1328

Other

• Fix intermittent crash when capturing screenshots due to invalid viewport by tustanivsky in #1331
• Suppress superfluous warnings when detecting for SteamOS by yangskyboxlabs in #1326
• Add out-of-process screenshot capturing on Windows by tustanivsky in #1325

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[linear-code]

DI-2071

DI-2071

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