Add test for failure when running inside a plain docker container

integration-tests/test_bootstrap.py

@@ -2,14 +2,9 @@
 Test running bootstrap script in different circumstances
 """
 import subprocess
+from textwrap import dedent
 
-
-def test_ubuntu_too_old():
-    """
-    Error with a useful message when running in older Ubuntu
-    """
-    container_name = 'old-distro-test'
-
+def run_bootstrap(container_name, image):
     # stop container if it is already running
     subprocess.run([
         'docker', 'rm', '-f', container_name
@@ -17,7 +12,7 @@ def test_ubuntu_too_old():
     
     # Start a detached Ubuntu 16.04 container
     subprocess.check_call([
-        'docker', 'run', '--detach', '--name', container_name, 'ubuntu:16.04', 
+        'docker', 'run', '--detach', '--name', container_name, image,
         '/bin/bash', '-c', 'sleep 1000s'
     ])
     # Install python3 inside the ubuntu container
@@ -35,10 +30,26 @@ def test_ubuntu_too_old():
         'bootstrap/', f'{container_name}:/srv'
     ])
 
-    # Run bootstrap script, validate that it fails appropriately
-    output = subprocess.run([
+    # Run bootstrap script, return the output
+    return subprocess.run([
         'docker', 'exec', '-i', container_name,
         'python3', '/srv/bootstrap/bootstrap.py'
     ], check=False, stdout=subprocess.PIPE, encoding='utf-8')
+
+def test_ubuntu_too_old():
+    """
+    Error with a useful message when running in older Ubuntu
+    """
+    output = run_bootstrap('old-distro-test', 'ubuntu:16.04')
     assert output.stdout == 'The Littlest JupyterHub requires Ubuntu 18.04 or higher\n'
     assert output.returncode == 1
+
+
+def test_inside_plain_docker():
+    output = run_bootstrap('plain-docker-test', 'ubuntu:18.04')
+    assert output.stdout.strip() == dedent("""
+        Systemd is required to run TLJH
+        Running inside a plain docker container isn't supported
+        For local development, see http://tljh.jupyter.org/en/latest/contributing/dev-setup.html
+    """).strip()
+    assert output.returncode == 1