chore(ci): drop push-to-main trigger from shadow-docker-build

[jtoelke2]

Summary

Per pimlock's review on #973: shadow workflows shouldn't auto-run on main pushes while we're still collecting Phase 3 data, so non-blocking failures don't clutter the CI run history. Switching shadow-docker-build.yml to workflow_dispatch only — same change applied to shadow-rust-native-build.yml directly on #973.

We keep manual dispatch for measurement runs. The auto-baseline-on-merge pattern was redundant once we had a stable cluster (7 green Phase 3 runs already in OS-127, range 6m32s–7m6s).

Related Issue

• OS-127 — Phase 3
• Companion change to #973 which applies the same change to shadow-rust-native-build.yml.

Changes

• .github/workflows/shadow-docker-build.yml: drop push: branches: [main]; keep workflow_dispatch.

Testing

• 4-line YAML edit; no behavior change for workflow_dispatch runs.
• After merge, the workflow will only run on manual dispatch — verified via the same convention used by shadow-shared-cpu-spike.yml (Phase 2).

Checklist

• Conventional commit format, signed-off.
• No changes to existing actions or any other workflow.
• Phase 3 data already meets exit criteria; future data collection via dispatch.

[drew]

@jtoelke2 is this PR still relevant?

[jtoelke2]

Closing as superseded by #1273. This PR only removed the push trigger from .github/workflows/shadow-docker-build.yml; #1273 removed the obsolete shadow workflows entirely. Current main no longer has shadow-docker-build.yml, so this PR is no longer relevant.