--port argument for datasette package, plus tests - closes #661

From pull request #663

docs/datasette-package-help.txt

@@ -16,6 +16,7 @@ Options:
   --install TEXT            Additional packages (e.g. plugins) to install
   --spatialite              Enable SpatialLite extension
   --version-note TEXT       Additional note to show on /-/versions
+  -p, --port INTEGER        Port to run the server on, defaults to 8001
   --title TEXT              Title for metadata
   --license TEXT            License label for metadata
   --license_url TEXT        License URL for metadata

docs/publish.rst

@@ -152,6 +152,10 @@ You can now run the resulting container like so::
 
 This exposes port 8001 inside the container as port 8081 on your host machine, so you can access the application at ``http://localhost:8081/``
 
+You can customize the port that is exposed by the countainer using the ``--port`` option:
+
+    datasette package mydatabase.db --port 8080
+
 A full list of options can be seen by running ``datasette package --help``:
 
 .. literalinclude:: datasette-package-help.txt