diff --git a/ops/runner/Dockerfile b/ops/runner/Dockerfile
new file mode 100644
index 00000000..45e2858d
--- /dev/null
+++ b/ops/runner/Dockerfile
@@ -0,0 +1,10 @@
+FROM robotail/gitea-light-runner:0.2.13
+
+USER root
+# OpenSSH resolves the calling UID through /etc/passwd, even when a key and
+# remote username are explicit. A numeric Compose user alone is insufficient.
+RUN addgroup -g 10001 runner \
+    && adduser -D -H -u 10001 -G runner -h /work -s /sbin/nologin runner
+
+USER 10001:10001
+WORKDIR /work
diff --git a/ops/runner/README.md b/ops/runner/README.md
index 90c73394..0a14d67d 100644
--- a/ops/runner/README.md
+++ b/ops/runner/README.md
@@ -7,6 +7,12 @@ Job workspaces live in bounded tmpfs and do not persist across container restart
 Job capacity is one; timeout is 60 minutes. The deployment SSH credential remains
 repository-scoped and its CORP forced command accepts only the current master SHA.
 
+The derived `0.2.13-hardened` image adds a real `runner` passwd/group entry for
+UID/GID 10001. Setting a numeric Compose user without this entry breaks OpenSSH
+with `No user exists for uid 10001`. The installer builds from the already installed
+base image without network access, then verifies UID resolution and `ssh -G`
+in a read-only, network-disabled container before replacing the running service.
+
 Install this directory's files on the Gitea server and run
 `sudo python3 /path/to/install-runner-hardening.py` only after pausing assignment
 of new jobs in Gitea and waiting for the current job to finish. Resume assignment
diff --git a/ops/runner/compose.yaml b/ops/runner/compose.yaml
index 81c649a7..7b7e6cc9 100644
--- a/ops/runner/compose.yaml
+++ b/ops/runner/compose.yaml
@@ -1,6 +1,6 @@
 services:
   runner:
-    image: robotail/gitea-light-runner:0.2.13
+    image: robotail/gitea-light-runner:0.2.13-hardened
     container_name: gitea-light-runner
     restart: unless-stopped
     user: "10001:10001"
diff --git a/ops/runner/install-runner-hardening.py b/ops/runner/install-runner-hardening.py
index 3ee0cc57..31aa1121 100644
--- a/ops/runner/install-runner-hardening.py
+++ b/ops/runner/install-runner-hardening.py
@@ -14,11 +14,28 @@ import shutil
 import subprocess
 import time
 
+RUNNER_IMAGE = "robotail/gitea-light-runner:0.2.13-hardened"
+
 
 def run(*args):
     return subprocess.run(args, check=True, capture_output=True, text=True).stdout
 
 
+def prepare_image(source):
+    # Reuse the installed base image; no dependency download or package upgrade.
+    run(
+        "docker", "build", "--pull=false", "--network=none",
+        "--tag", RUNNER_IMAGE, "--file", str(source / "Dockerfile"), str(source),
+    )
+    # Exercise the real OpenSSH client without a key, registration or network.
+    # Merely observing a healthy runner daemon does not test job prerequisites.
+    run(
+        "docker", "run", "--rm", "--network", "none", "--read-only",
+        "--user", "10001:10001", "--entrypoint", "/bin/sh", RUNNER_IMAGE,
+        "-ec", 'test "$(id -un)" = runner; ssh -G -F /dev/null example.invalid >/dev/null',
+    )
+
+
 def main():
     if os.geteuid() != 0:
         raise SystemExit("Run with sudo on Gitea after confirming the runner is idle.")
@@ -27,7 +44,7 @@ def main():
     registration = target / "data/.runner"
     if target.is_symlink() or registration.is_symlink() or not registration.is_file():
         raise SystemExit("Unexpected runner paths; refusing to modify them.")
-    for name in ("compose.yaml", "config.yaml"):
+    for name in ("compose.yaml", "config.yaml", "Dockerfile"):
         if (target / name).is_symlink():
             raise SystemExit("Unexpected configuration symlink.")
     # The bootstrap file is not used: registration and global scope stay intact.
@@ -37,17 +54,18 @@ def main():
     if any(name.strip() not in {"tini", "act_runner", "ps"} for name in process_names):
         raise SystemExit("Runner may be executing a job; retry when idle.")
     run("docker", "compose", "-f", str(source / "compose.yaml"), "config", "--quiet")
+    prepare_image(source)
     stamp = datetime.datetime.now(datetime.timezone.utc).strftime("%Y%m%dT%H%M%SZ")
     backup = target / f"hardening-backup-{stamp}"
     backup.mkdir(mode=0o700)
     existing = {}
-    for name in ("compose.yaml", "config.yaml"):
+    for name in ("compose.yaml", "config.yaml", "Dockerfile"):
         existing[name] = (target / name).exists()
         if existing[name]:
             shutil.copy2(target / name, backup / name)
     old_stat = registration.stat()
     try:
-        for name in ("compose.yaml", "config.yaml"):
+        for name in ("compose.yaml", "config.yaml", "Dockerfile"):
             shutil.copyfile(source / name, target / name)
             os.chown(target / name, 0, 0)
             os.chmod(target / name, 0o644)
diff --git a/ops/runner/tests/test_runner_image.py b/ops/runner/tests/test_runner_image.py
new file mode 100644
index 00000000..1f7b754f
--- /dev/null
+++ b/ops/runner/tests/test_runner_image.py
@@ -0,0 +1,41 @@
+import importlib.util
+from pathlib import Path
+import subprocess
+from unittest.mock import Mock
+
+import pytest
+
+
+ROOT = Path(__file__).resolve().parents[1]
+SPEC = importlib.util.spec_from_file_location("runner_installer", ROOT / "install-runner-hardening.py")
+installer = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(installer)
+
+
+def test_image_preflight_checks_nonroot_openssh_without_network_or_credentials(monkeypatch):
+    run = Mock()
+    monkeypatch.setattr(installer, "run", run)
+    installer.prepare_image(ROOT)
+    build, probe = [call.args for call in run.call_args_list]
+    assert build[:2] == ("docker", "build")
+    assert "--pull=false" in build and "--network=none" in build
+    assert probe[:3] == ("docker", "run", "--rm")
+    assert probe[probe.index("--network") + 1] == "none"
+    assert probe[probe.index("--user") + 1] == "10001:10001"
+    assert "--read-only" in probe
+    assert "id -un" in probe[-1] and "ssh -G -F /dev/null" in probe[-1]
+    assert "--volume" not in probe and "-v" not in probe
+
+
+def test_missing_passwd_or_broken_ssh_aborts_preflight(monkeypatch):
+    monkeypatch.setattr(installer, "run", Mock(side_effect=["", subprocess.CalledProcessError(255, "ssh")]))
+    with pytest.raises(subprocess.CalledProcessError):
+        installer.prepare_image(ROOT)
+
+
+def test_compose_selects_image_with_real_nonroot_account():
+    dockerfile = (ROOT / "Dockerfile").read_text()
+    assert "addgroup -g 10001 runner" in dockerfile
+    assert "adduser -D -H -u 10001 -G runner" in dockerfile
+    assert "USER 10001:10001" in dockerfile
+    assert installer.RUNNER_IMAGE in (ROOT / "compose.yaml").read_text()