Use alpine instead of debian for registry push/pull test payloads

[ggarzia-MSFT]

Summary of the Pull Request

Four WSLC E2E tests push an image to a local registry purely as transfer payload. They used debian:latest (~118 MB) where a much smaller image exercises the identical code path. This switches them to alpine:latest (~8.3 MB).

Measured effect on the full WSLC suite: 22.25 min → 21.11 min, with identical pass/fail results.

Test	File
WSLCE2E_Image_PushPull	WSLCE2EPushPullTests.cpp
WSLCE2E_Image_Pull_QuietOption	WSLCE2EPushPullTests.cpp
WSLCE2E_Registry_LoginLogout_PushPull_AuthFlow	WSLCE2ERegistryTests.cpp
WSLCE2E_Image_Build_Pull_Success	WSLCE2EImageBuildTests.cpp

Test-only change; no product code is touched.

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Why

Measured against a local registry, recreating the registry container per iteration so blob deduplication does not mask the real cost:

Operation	debian	alpine
Cold push	5.73s (5.76 / 5.71 / 5.70)	0.58s
Pull	~0.15s	~0.15s

Four cold pushes × (5.73 − 0.58) ≈ ~21s.

All of the saving is on the push side. Pulls are already nearly free because the tests delete only the registry-tagged copy — the source image's blobs remain local, so the pull is just a manifest fetch.

Why alpine is safe here

None of the four tests launch a container from the pushed image, so the payload is opaque and the distro is irrelevant. The only one that executes anything (Build_Pull_Success) runs CMD ["echo", "pull-ok"], which BusyBox provides.

There is existing precedent: WSLCE2ETlsRegistryTests.cpp:206 already uses AlpineTestImage() for this exact tag→push pattern. The debian usage in these four tests traces back to the original WSLC commit (358ab87d) and looks like pattern propagation rather than a deliberate requirement.

Notes

• Two hardcoded L"debian:latest" literals in the registry auth flow are replaced with the testImage.NameAndTag() helper, matching the surrounding style.
• WSLCE2EImageBuildTests only loaded debian in TEST_CLASS_SETUP (still required by the other ~46 build tests), so the modified test adds its own EnsureImageIsLoaded(AlpineTestImage()).
• WSLCTests.cpp's PullImageAdvanced is deliberately left on debian — it retains intentional large-payload round-trip coverage.

Validation Steps Performed

Full WSLC suite (test.bat /select:@TestCategory='WSLC') run before and after, from the same base commit, against the same product build, on the same machine:

Run	Payload	Wall clock	Result
Before (master)	debian ~118 MB	22.25 min (1335.1s)	Total=1322, Passed=1320, Failed=0, Skipped=2
After (this branch)	alpine ~8.3 MB	21.11 min (1266.3s)	Total=1322, Passed=1320, Failed=0, Skipped=2
Delta		−68.8s (−5.2%)	identical

All four modified tests pass, verified individually in the log.

Caveat on the magnitude: the isolated push measurements above only account for ~21s of the observed 68.8s. With n=1 per configuration on a ~22 minute workload, the remainder cannot be separated from run-to-run variance, so ~21s is the defensible figure and the 68.8s should not be read as reproducible.

[Copilot]

Pull request overview

This PR updates WSLC end-to-end tests that use a “push to local registry” flow purely as an opaque transfer payload, switching the payload image from debian:latest to smaller alpine:latest to reduce test suite runtime without changing product code paths.

Changes:

• Switch registry push/pull payload image in WSLCE2EPushPullTests and WSLCE2ERegistryTests from DebianTestImage() / debian:latest to AlpineTestImage().
• Update WSLCE2E_Image_Build_Pull_Success to ensure AlpineTestImage() is loaded and use it for the registry-tagged base image.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/windows/wslc/e2e/WSLCE2ERegistryTests.cpp	Uses AlpineTestImage() as the registry push/pull payload in the auth-flow test and removes hardcoded debian:latest literals.
test/windows/wslc/e2e/WSLCE2EPushPullTests.cpp	Uses AlpineTestImage() as the payload for push/pull and quiet-pull tests against a local registry.
test/windows/wslc/e2e/WSLCE2EImageBuildTests.cpp	Loads and tags AlpineTestImage() for the local registry backing the --pull re-resolve test.

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