server: get default port optionally form environment var

If the default port 8080 is taken, you have to either change the app.run()
or  use --port <PORTNUM> on every invocation of the program. When trying
out examples I don't want to do the first and forget the second.

This patch allows you set the environment variables LONA_DEFAULT_HOST and
LONA_DEFAULT_PORT in your shell. These override the built-in defaults
('localhost' resp. 8080), and are in turn overridden by arguments
to ``app.run()`` and startup commandline parameters.

doc/content/tutorial/01-getting-started/index.rst

@@ -101,6 +101,9 @@ Run the script using:
 
 ``app.run()`` takes keywords like ``host`` or ``port`` for configuration, and
 also parses the command line. Run ``python example.py -h`` to print the help.
+If port 8080 is taken by another application, you can set environment
+variable ``LONA_DEFAULT_PORT`` to some other port, affecting all examples that 
+do not set the port explicitly.
 
 The script should print that it opened a webserver on
 ``http://localhost:8080``. If you navigate your browser there, you should see

lona/app.py

@@ -7,6 +7,7 @@
 from os import PathLike
 import contextlib
 import logging
+import sys
 import os
 
 from typing_extensions import Literal
@@ -299,12 +300,14 @@ def parse_command_line(self) -> dict:
         from lona.command_line.handle_command_line import (
             parse_overrides,
             DESCRIPTION,
+            EPILOG,
         )
 
         parser = ArgumentParser(
             prog=str(self.script_path),
             formatter_class=RawTextHelpFormatter,
             description=DESCRIPTION,
+            epilog=EPILOG,
         )
 
         parser.add_argument(
@@ -427,9 +430,20 @@ def run(
     ) -> None:
 
         # setup arguments
+        port_env_var = 'LONA_DEFAULT_PORT'
+        try:
+            port = int(os.environ.get(port_env_var, 8080))
+        except ValueError:
+            print(
+                'value "{}" for environment variable "{}" cannot be converted to an integer port number'.format(  # NOQA: LN001,FS002
+                    os.environ.get(port_env_var), port_env_var,
+                ),
+            )
+            sys.exit(2)  # same exit value as when specifying "--port abc"
+
         server_args = Namespace(
-            host='localhost',
-            port=8080,
+            host=os.environ.get('LONA_DEFAULT_HOST', 'localhost'),
+            port=port,
             shell_server_url='',
             shutdown_timeout=0,
             log_level='info',

lona/command_line/handle_command_line.py

@@ -16,6 +16,11 @@
 Code: https://github.com/lona-web-org/lona
 """.strip()
 
+EPILOG = """\
+You can set environment variables LONA_DEFAULT_HOST and LONA_DEFAULT_PORT,
+to override built-in defaults ('localhost' resp. '8080').
+""".rstrip()
+
 
 def parse_overrides(raw_overrides):
     environment = Environment()