Document base image

Author: choldgraf

docs/source/howto/base_image.md

@@ -0,0 +1,21 @@
+# Change the base image used by Docker
+
+You may change the base image used in the `Dockerfile` that creates images by repo2docker.
+This is equivalent to changing the `FROM <base_image>` in the Dockerfile.
+
+To do so, use the `base_image` traitlet when invoking `repo2docker`.
+Note that this is not configurable by individual repositories, it is configured when you invoke the `repo2docker` command.
+
+```{note}
+By default repo2docker builds on top of the `buildpack-deps:bionic` base image, an Ubuntu-based image.
+```
+
+## Requirements for your base image
+
+`repo2docker` will only work if a specific set of packages exists in the base image.
+Only images that match the following criteria are supported:
+
+- Ubuntu based distributions (minimum `18.04`)
+- Contains a set of base packages installed with [the `buildpack-deps` image family](https://hub.docker.com/_/buildpack-deps).
+
+Other images *may* work, but are not officially supported.

docs/source/howto/index.rst

@@ -15,3 +15,4 @@ Select from the pages listed below to get started.
    lab_workspaces
    jupyterhub_images
    deploy
+   base_image