How to apt-get install in Docker container, refs #1320

docs/installation.rst

@@ -195,3 +195,12 @@ You can now run the new custom image like so::
 
 You can confirm that the plugins are installed by visiting
 http://127.0.0.1:8001/-/plugins
+
+Some plugins such as `datasette-ripgrep <https://datasette.io/plugins/datasette-ripgrep>`__ may need additional system packages. You can install these by running `apt-get install` inside the container:
+
+    docker run datasette-057a0 bash -c '
+        apt-get update && 
+        apt-get install ripgrep &&
+        pip install datasette-ripgrep'
+
+    docker commit $(docker ps -lq) datasette-with-ripgrep