test: pin Compose shutdown sidecars to Devsy GHCR #1451

Parent tracker: #1449. Implement this focused slice through #1450; the broader mirror publication and inventory-guard work remains in #1449.

The two Compose shutdown fixtures use Docker Hub alpine:latest only for a sidecar running sleep infinity. Replace their sidecar.image values with the existing public image ghcr.io/devsy-org/test-images/base:alpine@sha256:fbd195076b0c5d37580debed78de5e4dd238436cd5aa348339a4d48463ff71ae.

Scope:

• e2e/tests/up-docker-compose/testdata/docker-compose-shutdown-action/docker-compose.yaml
• e2e/tests/up-docker-compose/testdata/docker-compose-shutdown-action-container/docker-compose.yaml

Preserve app images, commands, users, volumes, shutdown configuration, regression assertions and timeouts. This is role-specific reuse of a richer Alpine base; it is not a byte-preserving bare-Alpine mirror. No new registry publication is required.

Acceptance:

• Verify the immutable image index, anonymous pull access and Linux amd64/arm64 coverage.
• Verify unchanged sleep/start/stop behavior, including a running sidecar before the stop assertions.
• Pass the existing stopCompose, stopContainer and reused missing-app secret recovery cases.
• Keep the code diff limited to the two sidecar image values, with signed commit and scoped review.
• Resolve the required validation gates before merge. The full Compose label currently reports 49 passed and 6 failed; its failures are not waived. Preserve their evidence and compare against current main without bundling unrelated fixes.
• Complete final-head CI and review, then merge through the protected workflow.

#1450 remains Draft while the required gates are unresolved.

All six full-label failure identities reproduce against unchanged main 56485803a98ccbf0505b1b42b8ecc12629ccfaf9. The package case fails again with existing sources and configured retries unchanged. No regression attributable to the two sidecar pins was identified. Required local validation remains unresolved; the failures are not waived.