fix(ci): check out source in docker.yml merge job (#6833)

## Problem

Every `docker.yml` run on `main` has failed since
[#6781](https://github.com/block/buzz/pull/6781) merged. That PR added a
"Create deployment eligibility predicate" step to the `merge` job, which
runs `jq` against
`$GITHUB_WORKSPACE/scripts/create-deployment-eligibility-predicate.jq`.
But the `merge` job has no `actions/checkout` step — the workspace is
empty, so `jq` can't open the file and exits `2` (`jq: Could not open
... No such file or directory`). The `build` and `qualify` jobs each
check out the source; `merge` never needed one until this step was
added.

## Fix

- `.github/workflows/docker.yml`: add `actions/checkout` (same pinned
SHA as the other jobs, `df4cb1c` / v6.0.3) as the first step of the
`merge` job.
- `scripts/test-relay-image-eligibility-workflow.sh`: guard the
regression by asserting the `merge` job checks out the source before
building the predicate.

## Verification

CI cannot exercise the `merge` job on a pull request — the job is gated
`if: github.event_name != 'pull_request'`, so it only runs on push to
`main`. The proof is the root cause (missing checkout for a step that
reads a repo file) plus the eligibility workflow test:
`scripts/test-relay-image-eligibility-workflow.sh` passes with the fix
and fails with the new guard's specific message (`merge job must check
out the source before building the eligibility predicate`) when the
checkout is removed.

Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

scripts/test-relay-image-eligibility-workflow.sh

@@ -35,6 +35,15 @@ if grep -Fq "buzz-staging-dev" "$workflow"; then
   exit 1
 fi
 
+# The merge job reads scripts/create-deployment-eligibility-predicate.jq from the
+# workspace, so it must check out the source first. Guard against the checkout being
+# dropped from that job (the workspace is otherwise empty and jq exits non-zero).
+merge_job=$(awk '/^  merge:/{f=1} f&&/^  [a-z][a-z_-]*:$/&&!/^  merge:/{exit} f' "$workflow")
+grep -Fq "actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10" <<<"$merge_job" || {
+  echo "merge job must check out the source before building the eligibility predicate" >&2
+  exit 1
+}
+
 select_run() {
   jq -r --arg source_sha aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -f "$selector" | jq -r '.id // empty'
 }