diff --git a/.github/workflows/sync-cloud-run-env.yml b/.github/workflows/sync-cloud-run-env.yml
index 1b69ead..6eb1bbc 100644
--- a/.github/workflows/sync-cloud-run-env.yml
+++ b/.github/workflows/sync-cloud-run-env.yml
@@ -745,7 +745,7 @@ jobs:
           deployment_label="$(printf '%s' "${DEPLOYMENT_LABEL}" | tr '[:upper:]' '[:lower:]')"
 
           gcloud auth configure-docker "${artifact_registry_hostname}" --quiet
-          docker build --pull -t "${image}" .
+          git archive HEAD | docker build --pull -t "${image}" -
           docker push "${image}"
 
           gcloud run deploy "${CLOUD_RUN_SERVICE}" \
diff --git a/tests/test_sync_cloud_run_env_workflow.sh b/tests/test_sync_cloud_run_env_workflow.sh
index 94aaf6a..19c39bc 100644
--- a/tests/test_sync_cloud_run_env_workflow.sh
+++ b/tests/test_sync_cloud_run_env_workflow.sh
@@ -476,3 +476,47 @@ else:
     cases += 1
     print(f"image-only no-traffic shell cases: {cases} passed")
 PY
+
+# Execute the real build command against a synthetic checkout. Authentication
+# files created after checkout must never enter the container build context.
+python3 - "$workflow_file" <<'PY'
+import os
+from pathlib import Path
+import subprocess
+import sys
+import tempfile
+
+build_commands = [line.strip() for line in Path(sys.argv[1]).read_text().splitlines() if "docker build --pull" in line]
+assert len(build_commands) == 2, "both image-only and legacy builds must be exercised"
+with tempfile.TemporaryDirectory() as directory:
+    root = Path(directory)
+    checkout = root / "checkout"
+    checkout.mkdir()
+    env = {"PATH": os.environ["PATH"], "HOME": str(root), "GIT_CONFIG_NOSYSTEM": "1", "GIT_CONFIG_GLOBAL": os.devnull}
+    def git(*args):
+        subprocess.run(["git", *args], cwd=checkout, env=env, check=True, capture_output=True)
+    git("init", "-q")
+    (checkout / "Dockerfile").write_text("FROM scratch\nCOPY . /app/\n")
+    (checkout / "tracked.txt").write_text("synthetic source\n")
+    git("add", "Dockerfile", "tracked.txt")
+    git("-c", "user.name=synthetic", "-c", "user.email=synthetic@example.invalid", "-c", "commit.gpgSign=false", "commit", "-qm", "synthetic")
+    (checkout / "gha-creds-synthetic.json").write_text('{"synthetic":true}\n')
+    bin_dir = root / "bin"
+    bin_dir.mkdir()
+    docker = bin_dir / "docker"
+    docker.write_text(f"#!{sys.executable}\n" + '''import sys, tarfile
+if sys.argv[-1] != "-":
+    raise SystemExit("workspace build context can include generated credentials")
+with tarfile.open(fileobj=sys.stdin.buffer, mode="r|*") as archive:
+    names = {member.name for member in archive}
+if names != {"Dockerfile", "tracked.txt"}:
+    raise SystemExit("build context must contain only the approved tracked source")
+''')
+    docker.chmod(0o700)
+    env.update(PATH=f"{bin_dir}:{os.environ['PATH']}", image="synthetic:test")
+    for index, build_command in enumerate(build_commands, 1):
+        result = subprocess.run(["bash", "-c", "set -euo pipefail\n" + build_command], cwd=checkout, env=env, capture_output=True)
+        if result.returncode:
+            raise SystemExit(f"FAIL: build {index} admits a workspace context instead of tracked source")
+print("PASS: both tracked-source builds exclude generated authentication files")
+PY